Check Engine Code Lookup
Search any OBD-II trouble code for what it actually means, what usually causes it, how serious it is, and what the repair typically costs. 72 of the most common codes, in plain English.
Most searched codes
Showing 72 of 72 codes
P0087Fuel Rail / System Pressure Too LowHigh
The fuel system isn't building the pressure the computer expects, so the engine is being starved of fuel under load.
Common causes
- Clogged fuel filter
- Failing fuel pump
- Restricted fuel line
- Faulty fuel pressure regulator
- Bad fuel rail pressure sensor
Can you keep driving?
Short trips only. The engine can stall or lose power without warning, especially under acceleration.
Typical repair cost
$150 – $1,200
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0088Fuel Rail / System Pressure Too HighHigh
Fuel pressure is above the target, which can flood the cylinders and wash oil off the cylinder walls.
Common causes
- Stuck fuel pressure regulator
- Blocked fuel return line
- Faulty rail pressure sensor
- Fuel pump control module fault
Can you keep driving?
Not recommended. Over-fuelling can damage the catalytic converter and dilute your engine oil.
Typical repair cost
$200 – $1,100
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0101Mass Air Flow (MAF) Circuit Range / PerformanceModerate
The mass air flow sensor is reporting an airflow figure that doesn't match what the engine should be breathing at that moment.
Common causes
- Dirty or contaminated MAF sensor
- Air filter installed incorrectly
- Intake leak after the sensor
- Split intake boot
- Failing MAF sensor
Can you keep driving?
Yes, but expect rough idle, hesitation and worse fuel economy. Often fixed by cleaning the sensor.
Typical repair cost
$90 – $450
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0102Mass Air Flow (MAF) Circuit Low InputModerate
The MAF sensor signal is far lower than possible — usually a wiring problem or a dead sensor.
Common causes
- Damaged MAF wiring or connector
- Failed MAF sensor
- Severely clogged air filter
- Blocked intake
Can you keep driving?
Yes, short term. The engine will run on a backup fuel map — expect poor power and high fuel use.
Typical repair cost
$90 – $400
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0106MAP / Barometric Pressure Circuit Range / PerformanceModerate
The manifold pressure sensor reading doesn't line up with engine load, so the computer can't calculate the right fuel mix.
Common causes
- Vacuum leak
- Clogged MAP sensor port
- Failing MAP sensor
- Damaged vacuum hose
Can you keep driving?
Yes, but the engine may idle roughly and shift oddly on automatics.
Typical repair cost
$90 – $350
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0113Intake Air Temperature Sensor 1 Circuit HighLow
The intake air temperature signal is reading implausibly high — normally an open circuit rather than genuinely hot air.
Common causes
- Failed IAT sensor
- Broken or corroded wiring
- Loose connector
- Sensor contaminated with oil
Can you keep driving?
Yes. Minor driveability effect, but it will keep the light on and fail emissions.
Typical repair cost
$80 – $250
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0121Throttle / Pedal Position Sensor A Circuit Range / PerformanceHigh
The throttle position the computer sees doesn't agree with the airflow or pedal input it expects.
Common causes
- Dirty throttle body
- Worn throttle position sensor
- Damaged wiring
- Failing accelerator pedal sensor
Can you keep driving?
Drive with caution. Many vehicles drop into reduced-power 'limp' mode, which is dangerous when merging.
Typical repair cost
$120 – $650
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0128Coolant Thermostat Below Regulating TemperatureLow
The engine isn't reaching normal operating temperature in the expected time — nearly always a thermostat stuck open.
Common causes
- Thermostat stuck open
- Low coolant level
- Faulty coolant temperature sensor
- Cooling fan running constantly
Can you keep driving?
Yes. You'll see worse fuel economy and weak heat from the vents in winter.
Typical repair cost
$150 – $500
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0171System Too Lean (Bank 1)Moderate
The engine is getting more air than fuel and the computer has run out of room to compensate. One of the most common check engine codes there is.
Common causes
- Vacuum leak
- Dirty MAF sensor
- Failing fuel pump or clogged filter
- Leaking intake manifold gasket
- Faulty PCV valve or hose
- Weak fuel injectors
Can you keep driving?
Yes, in the short term. Left alone, a lean mixture runs hot and can eventually damage pistons and the catalytic converter.
Typical repair cost
$100 – $700
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0172System Too Rich (Bank 1)Moderate
The engine is getting more fuel than it can burn, so unburnt fuel is going out the exhaust.
Common causes
- Leaking fuel injector
- Failed fuel pressure regulator
- Dirty MAF sensor
- Clogged air filter
- Faulty oxygen sensor
- Leaking evaporative purge valve
Can you keep driving?
Yes, short term. Running rich destroys catalytic converters and thins the engine oil with fuel.
Typical repair cost
$100 – $800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0174System Too Lean (Bank 2)Moderate
Same as P0171 but on the engine's second cylinder bank. Seeing P0171 and P0174 together points to a shared cause like the MAF sensor or a big vacuum leak.
Common causes
- Vacuum leak
- Dirty MAF sensor
- Intake manifold gasket leak
- Low fuel pressure
- Failing PCV system
Can you keep driving?
Yes, in the short term, but book it in — lean running is hard on the engine.
Typical repair cost
$100 – $700
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0175System Too Rich (Bank 2)Moderate
The second cylinder bank is running with too much fuel in the mixture.
Common causes
- Leaking injectors on bank 2
- High fuel pressure
- Contaminated MAF sensor
- Faulty upstream oxygen sensor
Can you keep driving?
Yes, short term. Expect a fuel smell, black smoke and poor economy.
Typical repair cost
$100 – $800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0191Fuel Rail Pressure Sensor Circuit Range / PerformanceModerate
The fuel rail pressure sensor is giving a reading the computer doesn't believe.
Common causes
- Failing rail pressure sensor
- Wiring or connector fault
- Genuine fuel pressure problem
- Fuel pump control issue
Can you keep driving?
Yes, cautiously. Hard starting and stalling are common until it's fixed.
Typical repair cost
$150 – $600
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0299Turbocharger / Supercharger UnderboostHigh
The turbo isn't producing the boost pressure the computer commanded, so the engine feels flat.
Common causes
- Boost leak in pipes or intercooler
- Stuck wastegate or variable-vane mechanism
- Failing turbo
- Blocked air filter
- Faulty boost pressure sensor
Can you keep driving?
Gently, and not far. A failing turbo can shed debris straight into the engine.
Typical repair cost
$150 – $2,500
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P2279Intake Air System LeakModerate
Unmetered air is getting into the intake after the MAF sensor, upsetting the fuel mixture.
Common causes
- Split intake boot
- Loose hose clamp
- Cracked vacuum line
- Leaking intake gasket
- Faulty PCV hose
Can you keep driving?
Yes. Often a cheap fix once the leak is found — smoke testing is the fastest way.
Typical repair cost
$90 – $450
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0300Random / Multiple Cylinder Misfire DetectedHigh
The engine is misfiring across more than one cylinder, or the computer can't pin it to a single one. Usually something all cylinders share — fuel, spark or air.
Common causes
- Worn spark plugs
- Failing ignition coils
- Vacuum leak
- Low fuel pressure
- Clogged EGR valve
- Bad fuel batch
Can you keep driving?
Only if the light is steady. A flashing check engine light means raw fuel is reaching the catalytic converter — pull over and stop.
Typical repair cost
$120 – $1,200
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0301Cylinder 1 Misfire DetectedHigh
Cylinder 1 isn't burning its fuel properly — the code names the exact cylinder, which makes diagnosis much faster.
Common causes
- Bad spark plug
- Failed ignition coil
- Faulty fuel injector
- Low compression
- Vacuum leak at that cylinder
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0302Cylinder 2 Misfire DetectedHigh
Cylinder 2 is misfiring. Swapping the coil and plug with a neighbouring cylinder is the classic way to confirm the cause.
Common causes
- Bad spark plug
- Failed ignition coil
- Faulty fuel injector
- Low compression
- Damaged wiring
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0303Cylinder 3 Misfire DetectedHigh
Cylinder 3 is misfiring, causing a rough idle and a noticeable loss of power.
Common causes
- Bad spark plug
- Failed ignition coil
- Clogged injector
- Low compression
- Head gasket leak
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0304Cylinder 4 Misfire DetectedHigh
Cylinder 4 isn't firing cleanly. On a four-cylinder engine this is very obvious — the car will shake at idle.
Common causes
- Bad spark plug
- Failed ignition coil
- Faulty injector
- Burnt valve
- Vacuum leak
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0305Cylinder 5 Misfire DetectedHigh
Cylinder 5 is misfiring — found on V6, V8 and inline-six engines.
Common causes
- Bad spark plug
- Failed ignition coil
- Faulty injector
- Low compression
- Wiring fault
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $950
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0306Cylinder 6 Misfire DetectedHigh
Cylinder 6 is misfiring. On V-engines the rear bank is harder to reach, so labor costs more.
Common causes
- Bad spark plug
- Failed ignition coil
- Faulty injector
- Low compression
- Intake gasket leak
Can you keep driving?
Steady light: short trips only. Flashing light: stop driving.
Typical repair cost
$120 – $1,000
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0316Misfire Detected on Startup (First 1,000 Revolutions)Moderate
The engine misfires briefly on cold start then settles. Often an early warning of a fuel or ignition part on its way out.
Common causes
- Worn spark plugs
- Weak ignition coil
- Leaking injector
- Low fuel pressure at start
- Carbon build-up on valves
Can you keep driving?
Yes, but it usually progresses into a full misfire code if left alone.
Typical repair cost
$120 – $800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0325Knock Sensor 1 Circuit (Bank 1)Moderate
The knock sensor that listens for pre-ignition has failed or lost its signal, so the computer pulls timing as a precaution.
Common causes
- Failed knock sensor
- Damaged wiring harness
- Corroded connector
- Genuine engine knock from bad fuel
Can you keep driving?
Yes, but the engine will feel sluggish because timing is retarded to protect it.
Typical repair cost
$200 – $800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0335Crankshaft Position Sensor A CircuitCritical
The computer has lost the crankshaft position signal it uses to time spark and fuel. Without it the engine may not start at all.
Common causes
- Failed crankshaft position sensor
- Damaged reluctor ring
- Wiring or connector damage
- Metal debris on the sensor tip
Can you keep driving?
No. Expect sudden stalling and no-start conditions — this one strands people.
Typical repair cost
$180 – $700
Including diagnosis. Stop driving where it's safe to do so. This can destroy the engine or leave you stranded.
P0340Camshaft Position Sensor A Circuit (Bank 1)High
The camshaft position signal is missing or erratic, so the computer can't sequence the injectors correctly.
Common causes
- Failed camshaft position sensor
- Wiring fault
- Stretched timing chain
- Damaged tone ring
Can you keep driving?
Barely. Hard starting, stalling and long crank times are typical.
Typical repair cost
$180 – $700
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0350Ignition Coil Primary / Secondary CircuitHigh
A generic ignition coil circuit fault — the computer sees a coil that isn't switching the way it should.
Common causes
- Failed ignition coil
- Broken coil connector
- Damaged wiring
- Faulty ignition control module
Can you keep driving?
Short distances only, and not if the light is flashing.
Typical repair cost
$150 – $700
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0011Camshaft Position 'A' Timing Over-Advanced (Bank 1)Moderate
Variable valve timing is more advanced than commanded — usually an oil flow problem in the VVT system rather than a broken part.
Common causes
- Dirty or low engine oil
- Clogged VVT oil control valve screen
- Failed camshaft actuator solenoid
- Worn timing chain
- Faulty cam phaser
Can you keep driving?
Yes, short term. Start with an oil and filter change — it fixes this code surprisingly often.
Typical repair cost
$90 – $1,200
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0016Crankshaft / Camshaft Position Correlation (Bank 1 Sensor A)High
The crank and cam sensors disagree about where the engine is in its cycle — the timing between them has drifted out of spec.
Common causes
- Stretched or jumped timing chain
- Failed cam phaser
- Worn timing chain guides
- Low oil pressure
- Faulty cam or crank sensor
Can you keep driving?
No. On interference engines a jumped chain can bend valves and write the engine off.
Typical repair cost
$300 – $2,500
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0420Catalyst System Efficiency Below Threshold (Bank 1)Moderate
The downstream oxygen sensor says the catalytic converter isn't cleaning the exhaust as well as it should. The single most-searched check engine code — and not always the converter's fault.
Common causes
- Worn catalytic converter
- Failing downstream oxygen sensor
- Exhaust leak before the converter
- Engine running rich or burning oil
- Unresolved misfire that poisoned the converter
Can you keep driving?
Yes. It won't strand you, but it's an automatic emissions test failure — and the fix gets pricier if the underlying cause is a misfire.
Typical repair cost
$200 – $2,400
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0430Catalyst System Efficiency Below Threshold (Bank 2)Moderate
Same as P0420 but for the second bank of cylinders on a V6 or V8.
Common causes
- Worn catalytic converter on bank 2
- Failing downstream oxygen sensor
- Exhaust leak
- Rich running condition
Can you keep driving?
Yes, but it will fail emissions testing.
Typical repair cost
$200 – $2,400
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0401EGR Flow Insufficient DetectedModerate
The exhaust gas recirculation system isn't flowing enough exhaust back into the intake, usually because carbon has blocked the passages.
Common causes
- Carbon-clogged EGR passages
- Stuck EGR valve
- Failed EGR position sensor
- Blocked EGR cooler
- Vacuum supply leak
Can you keep driving?
Yes. You may notice pinging under load and slightly worse economy.
Typical repair cost
$150 – $800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0403EGR Control Circuit MalfunctionModerate
The electrical circuit that operates the EGR valve has a fault.
Common causes
- Failed EGR valve solenoid
- Open or shorted wiring
- Corroded connector
- PCM driver circuit fault
Can you keep driving?
Yes, though emissions will be out of spec.
Typical repair cost
$150 – $700
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0440Evaporative Emission Control System FaultLow
A general fault in the system that stops fuel vapour escaping from the tank into the atmosphere.
Common causes
- Loose or faulty fuel cap
- Cracked EVAP hose
- Failed purge valve
- Leaking charcoal canister
Can you keep driving?
Yes, completely. Check the fuel cap first — it costs nothing and fixes this often.
Typical repair cost
$20 – $600
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0442Evaporative Emission System Leak Detected (Small Leak)Low
A small vapour leak somewhere in the fuel tank and EVAP plumbing — roughly the size of a 0.040 inch hole.
Common causes
- Fuel cap not tightened or seal perished
- Small crack in an EVAP hose
- Leaking purge or vent valve
- Pinhole in the charcoal canister
Can you keep driving?
Yes. No driveability effect, but it will fail an emissions test.
Typical repair cost
$20 – $500
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0446Evaporative Emission System Vent Control CircuitLow
The vent valve that lets the EVAP system breathe isn't responding correctly to commands.
Common causes
- Failed vent control valve
- Blocked vent filter
- Wiring or connector damage
- Spider or debris blocking the vent line
Can you keep driving?
Yes. You may notice the fuel pump clicking off repeatedly when filling up.
Typical repair cost
$120 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0455Evaporative Emission System Leak Detected (Gross Leak)Low
A large vapour leak. On most cars the very first thing to check is whether the fuel cap is on properly.
Common causes
- Missing or loose fuel cap
- Disconnected EVAP hose
- Failed purge valve stuck open
- Cracked charcoal canister
- Damaged filler neck
Can you keep driving?
Yes. You may smell fuel vapour, particularly after filling up.
Typical repair cost
$20 – $550
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0456Evaporative Emission System Leak Detected (Very Small Leak)Low
A very small vapour leak, around a 0.020 inch hole. These are fiddly to find and usually need a smoke test.
Common causes
- Fuel cap seal
- Hairline crack in an EVAP line
- Slightly leaking vent valve
- Perished O-ring at the canister
Can you keep driving?
Yes. Purely an emissions issue.
Typical repair cost
$90 – $500
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P2096Post Catalyst Fuel Trim System Too Lean (Bank 1)Moderate
The oxygen sensor after the converter is reading lean, which often means an exhaust leak rather than a fuel problem.
Common causes
- Exhaust leak near the downstream sensor
- Failing downstream oxygen sensor
- Worn catalytic converter
- Genuine lean running condition
Can you keep driving?
Yes. Have the exhaust checked for leaks before anyone sells you a converter.
Typical repair cost
$150 – $1,200
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P2097Post Catalyst Fuel Trim System Too Rich (Bank 1)Moderate
The sensor after the converter is reading rich, so more fuel is getting through than expected.
Common causes
- Leaking fuel injector
- Failing downstream oxygen sensor
- Faulty purge valve
- Damaged catalytic converter
Can you keep driving?
Yes, short term. Rich running shortens converter life.
Typical repair cost
$150 – $1,200
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P2002Diesel Particulate Filter Efficiency Below ThresholdHigh
On a diesel, the particulate filter isn't trapping soot properly or is so blocked the system has given up regenerating it.
Common causes
- Clogged DPF from too many short trips
- Failed differential pressure sensor
- Faulty EGR causing excess soot
- Injector problem
- Cracked DPF substrate
Can you keep driving?
Limited. Many diesels drop into limp mode; a blocked DPF can turn into a very expensive replacement.
Typical repair cost
$300 – $3,500
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0131O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)Moderate
The upstream oxygen sensor is reporting a persistently low voltage — either a genuine lean condition or a dead sensor.
Common causes
- Failed oxygen sensor
- Exhaust leak upstream
- Wiring short to ground
- Genuine lean running condition
Can you keep driving?
Yes, but fuel economy suffers and the converter is at risk over time.
Typical repair cost
$150 – $500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0133O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)Moderate
The upstream oxygen sensor is still working but has become lazy, switching too slowly for accurate fuel control.
Common causes
- Aged oxygen sensor
- Silicone or oil contamination on the sensor
- Exhaust leak
- Poor connection at the sensor plug
Can you keep driving?
Yes. Classic wear item — most sensors are due around 100,000 miles.
Typical repair cost
$150 – $500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0135O2 Sensor Heater Circuit (Bank 1, Sensor 1)Low
The internal heater that brings the oxygen sensor up to temperature quickly has failed, so the engine stays in open loop longer.
Common causes
- Failed sensor heater element
- Blown fuse
- Damaged wiring or connector
- Relay fault
Can you keep driving?
Yes. Slightly worse cold-start economy, and it will fail emissions.
Typical repair cost
$150 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0137O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)Low
The oxygen sensor after the catalytic converter is reading low voltage.
Common causes
- Failed downstream oxygen sensor
- Exhaust leak near the sensor
- Wiring damage from road debris
- Corroded connector
Can you keep driving?
Yes. Mostly an emissions monitoring issue rather than a driveability one.
Typical repair cost
$150 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0141O2 Sensor Heater Circuit (Bank 1, Sensor 2)Low
The heater inside the downstream oxygen sensor has failed.
Common causes
- Failed heater element
- Blown fuse
- Broken wiring under the car
- Bad ground
Can you keep driving?
Yes. Replace it before your next emissions test.
Typical repair cost
$150 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P2195O2 Sensor Signal Stuck Lean (Bank 1, Sensor 1)Moderate
The upstream sensor signal is frozen at a lean reading instead of switching back and forth as it should.
Common causes
- Failed oxygen sensor
- Vacuum or exhaust leak
- Low fuel pressure
- Contaminated sensor
Can you keep driving?
Yes, short term. Check for leaks before replacing the sensor.
Typical repair cost
$150 – $600
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0500Vehicle Speed Sensor A MalfunctionModerate
The computer isn't getting a reliable road speed signal, which affects the speedometer, cruise control and shift timing.
Common causes
- Failed vehicle speed sensor
- Damaged wiring
- Faulty ABS wheel speed sensor
- Bad instrument cluster connection
Can you keep driving?
Yes, cautiously. Cruise control and sometimes ABS will be disabled.
Typical repair cost
$150 – $500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0505Idle Air Control System MalfunctionModerate
The engine can't hold a stable idle because the idle air control system isn't metering air correctly.
Common causes
- Carbon-fouled throttle body
- Failed idle air control valve
- Vacuum leak
- Throttle body needs relearning after cleaning
Can you keep driving?
Yes, though the car may stall at stops. Cleaning the throttle body often fixes it.
Typical repair cost
$100 – $500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0506Idle Air Control System RPM Lower Than ExpectedLow
The engine is idling below its target speed, which is why it may feel like it's about to stall.
Common causes
- Dirty throttle body
- Carbon build-up on the intake valves
- Failing idle air control valve
- High electrical load at idle
Can you keep driving?
Yes, though stalling at junctions is a nuisance and can be unsafe.
Typical repair cost
$100 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0507Idle Air Control System RPM Higher Than ExpectedLow
The engine is idling faster than commanded — almost always unmetered air sneaking in somewhere.
Common causes
- Vacuum leak
- Sticking throttle plate
- Failed PCV valve
- Leaking intake gasket
- Throttle body needs relearning
Can you keep driving?
Yes. A high idle wears the brakes and torque converter over time.
Typical repair cost
$100 – $450
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0521Engine Oil Pressure Sensor / Switch Range / PerformanceCritical
The oil pressure reading is outside the expected range. It might be the sensor — or it might be genuinely low oil pressure, which is far more serious.
Common causes
- Failed oil pressure sensor
- Low oil level
- Worn oil pump
- Clogged oil pickup screen
- Wrong oil viscosity
Can you keep driving?
Check your oil level immediately. If pressure really is low, driving will destroy the engine — treat it as serious until proven to be just the sensor.
Typical repair cost
$120 – $2,500
Including diagnosis. Stop driving where it's safe to do so. This can destroy the engine or leave you stranded.
P0562System Voltage LowHigh
Charging system voltage is below the minimum the computer expects.
Common causes
- Failing alternator
- Worn drive belt or tensioner
- Bad battery
- Corroded battery terminals
- Poor ground connection
Can you keep driving?
Not far. Once the battery drains the car will die and won't restart.
Typical repair cost
$150 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0563System Voltage HighHigh
Charging voltage is too high, which cooks the battery and can damage electronics.
Common causes
- Failed voltage regulator in the alternator
- Faulty alternator
- Bad ground connection
- Wrong battery type fitted
Can you keep driving?
Not recommended. Overcharging can boil a battery and take out expensive modules.
Typical repair cost
$200 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0603Internal Control Module Keep Alive Memory (KAM) ErrorModerate
The engine computer lost its stored adaptive memory — often after a battery disconnect, sometimes from an internal fault.
Common causes
- Battery disconnected or gone flat
- Poor battery ground
- Water damage to the ECM
- Internal ECM failure
Can you keep driving?
Usually yes. Clear the code and see if it returns before condemning the computer.
Typical repair cost
$100 – $1,500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0606ECM / PCM Processor FaultHigh
The engine control module has detected an internal processor fault in itself.
Common causes
- Failed engine control module
- Water ingress into the module
- Bad power or ground supply
- Corrupted software after a failed update
Can you keep driving?
No. Expect erratic behaviour, limp mode or a no-start.
Typical repair cost
$400 – $2,000
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0463Fuel Level Sensor Circuit High InputLow
The fuel gauge sender is reporting an impossible level — usually a failed sender inside the tank.
Common causes
- Failed fuel level sending unit
- Wiring short
- Corroded connector at the tank
- Instrument cluster fault
Can you keep driving?
Yes, but you can't trust the fuel gauge — track mileage between fill-ups instead.
Typical repair cost
$200 – $800
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
P0223Throttle / Pedal Position Sensor B Circuit HighHigh
The second throttle or pedal position signal is reading too high, and the two sensors no longer agree.
Common causes
- Failed accelerator pedal sensor
- Faulty throttle body
- Chafed wiring
- Poor connector contact
Can you keep driving?
With caution — reduced power mode is likely.
Typical repair cost
$150 – $700
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0201Injector Circuit / Open — Cylinder 1High
The electrical circuit to the number 1 fuel injector is open, so that cylinder isn't being fuelled.
Common causes
- Failed fuel injector
- Broken injector connector
- Chafed or rodent-damaged wiring
- PCM injector driver failure
Can you keep driving?
Short trips only. An unfuelled cylinder dumps raw air through the converter.
Typical repair cost
$180 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0700Transmission Control System (Malfunction Indicator Request)Moderate
This is not a fault in itself — it's the transmission computer telling the engine computer to turn the light on. You need to scan for the transmission-specific code behind it.
Common causes
- A stored transmission code such as P0730 or P0741
- Low or burnt transmission fluid
- Failing shift solenoid
- Faulty transmission control module
- Wiring fault to the transmission
Can you keep driving?
Depends entirely on the underlying code. Get a scan that reads transmission modules, not just a basic code reader.
Typical repair cost
$100 – $3,500
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0705Transmission Range Sensor CircuitModerate
The transmission doesn't know which gear position you've selected, which can stop the car starting in Park.
Common causes
- Failed neutral safety / range switch
- Misadjusted shift linkage
- Damaged wiring
- Internal TCM fault
Can you keep driving?
Yes, but the car may not start, may not shift properly, and reverse lights may not work.
Typical repair cost
$180 – $700
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0715Input / Turbine Speed Sensor CircuitHigh
The transmission can't measure how fast its input shaft is spinning, so it can't time shifts correctly.
Common causes
- Failed input speed sensor
- Low transmission fluid
- Damaged wiring
- Internal transmission wear
Can you keep driving?
Cautiously. Harsh or erratic shifting is common and hard on the transmission.
Typical repair cost
$200 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0720Output Speed Sensor CircuitHigh
The sensor measuring transmission output shaft speed has failed or lost signal.
Common causes
- Failed output speed sensor
- Damaged wiring or connector
- Debris on the sensor tip
- Low fluid level
Can you keep driving?
Cautiously. Speedometer errors and poor shift quality are typical.
Typical repair cost
$200 – $900
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0730Incorrect Gear RatioHigh
The transmission's actual gear ratio doesn't match the gear it thinks it selected — meaning it's slipping internally.
Common causes
- Low or degraded transmission fluid
- Worn clutch packs
- Failing shift solenoid
- Clogged valve body
- Internal mechanical wear
Can you keep driving?
Minimise driving. A slipping transmission wears fast, and a fluid service now may save a rebuild later.
Typical repair cost
$200 – $4,500
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
P0740Torque Converter Clutch CircuitModerate
The torque converter lock-up clutch circuit has a fault, so the converter may not lock at cruising speed.
Common causes
- Failed TCC solenoid
- Low or dirty transmission fluid
- Valve body wear
- Failing torque converter
- Wiring fault
Can you keep driving?
Yes, but expect worse highway fuel economy and higher transmission temperatures.
Typical repair cost
$200 – $1,800
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0741Torque Converter Clutch Circuit Performance / Stuck OffModerate
The torque converter clutch is being commanded to lock but isn't locking — you may feel a shudder around 40–50 mph.
Common causes
- Worn torque converter clutch
- Contaminated transmission fluid
- Stuck TCC solenoid
- Valve body wear
- Internal leak
Can you keep driving?
Yes, but heat builds up in the transmission. A fluid change sometimes clears it — try that before a converter.
Typical repair cost
$200 – $2,200
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P0755Shift Solenoid B MalfunctionHigh
One of the electrical valves that routes fluid to change gear isn't responding correctly.
Common causes
- Failed shift solenoid
- Dirty transmission fluid blocking the solenoid
- Wiring or connector fault
- Valve body damage
Can you keep driving?
Cautiously. The transmission may be stuck in one gear (limp mode) to protect itself.
Typical repair cost
$250 – $1,500
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
U0100Lost Communication With ECM / PCM 'A'High
Another module has stopped hearing from the engine computer over the vehicle's data network. Often a wiring or power fault rather than a dead computer.
Common causes
- Damaged CAN bus wiring
- Blown fuse to the ECM
- Corroded connector
- Failed engine control module
- Weak battery causing voltage dropouts
Can you keep driving?
No. Multiple systems will misbehave and the car may not start.
Typical repair cost
$150 – $2,000
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
U0101Lost Communication With Transmission Control ModuleHigh
The network link to the transmission computer has dropped out.
Common causes
- CAN bus wiring damage
- Blown TCM fuse
- Water in a connector
- Failed transmission control module
Can you keep driving?
Cautiously — the transmission will default to limp mode, usually one fixed gear.
Typical repair cost
$150 – $2,000
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
U0121Lost Communication With ABS Control ModuleHigh
The anti-lock brake module has fallen off the data network, so ABS and stability control are offline.
Common causes
- Blown ABS fuse
- Damaged wiring to the ABS module
- Corroded connector
- Failed ABS module
Can you keep driving?
Your normal brakes still work, but ABS, traction and stability control are disabled. Fix it before bad weather.
Typical repair cost
$150 – $1,600
Including diagnosis. Book a mechanic now. Continued driving risks damage that costs far more than the fix.
U0155Lost Communication With Instrument Panel ClusterModerate
The dashboard cluster isn't communicating with the rest of the car's network.
Common causes
- Blown fuse
- Loose cluster connector
- CAN wiring fault
- Failed instrument cluster
Can you keep driving?
Yes, but your gauges and warning lights may be wrong or dead — which hides other problems.
Typical repair cost
$150 – $1,200
Including diagnosis. Get it looked at within a week or two — ignoring it usually makes the repair bigger.
P1000OBD System Readiness Test Not CompleteLow
Not a fault. Common on Fords and Mazdas after the battery has been disconnected or codes cleared — the self-tests simply haven't finished yet.
Common causes
- Codes recently cleared
- Battery recently disconnected
- Not enough mixed driving since the reset
- An unresolved fault preventing tests completing
Can you keep driving?
Yes, completely. Drive a mix of city and highway for 50–100 miles and it should clear on its own.
Typical repair cost
$0 – $120
Including diagnosis. Not urgent, but it won't clear itself and it will fail an emissions test.
Now find a mechanic near you
An estimate is only half the job. Hidden Mechanics connects you with trustworthy, home-based mechanics who charge less than a dealership — and it's completely free for customers to search.
What your check engine light is actually telling you
When the check engine light comes on, your car has stored a diagnostic trouble code — a five-character identifier like P0420 or P0301 that points to which system reported a problem. Reading that code is the first step, but the code itself is a symptom, not a diagnosis. P0171 tells you the engine is running lean; it doesn't tell you whether that's a cracked vacuum hose, a dirty airflow sensor or a failing fuel pump. Knowing what the code means is what stops you from being sold a part you didn't need.
How to read the code
The first letter names the system: P for powertrain (engine and transmission), B for body, C for chassis and U for network and module communication. The digit after it tells you whether the code is generic across all manufacturers (0) or specific to your carmaker (1). The remaining three digits identify the exact fault. So P0301 breaks down as: powertrain, generic, cylinder 1 misfire.
Steady light or flashing light
This distinction matters more than the code number. A steady light means the computer has logged a fault it wants looked at, but the car is generally safe to drive to a shop. A flashing light means an active misfire is sending unburnt fuel into the catalytic converter — a component that can cost over $2,000 — and it can be ruined in a matter of minutes. If the light is flashing, stop driving as soon as it's safe to do so.
Before you spend money on parts
Two rules save more money than anything else. First, check the cheap things: a loose fuel cap genuinely does cause P0455, and a $12 oil change has cleared more than a few P0011 codes. Second, be suspicious of any shop that quotes a repair straight off the code without testing. Codes narrow the search; they don't end it. A proper diagnosis costs $90–$150 and is almost always cheaper than replacing the wrong part.
Found your code? Check what the fix should cost with the car repair cost estimator, or see whether the repair is worth it at all with the repair or replace calculator. Many of these codes trace back to skipped maintenance — the maintenance schedule checker will show what your mileage is due for. When you're ready for a real diagnosis, find a mechanic near you.
Frequently asked questions
P0420 means 'Catalyst System Efficiency Below Threshold (Bank 1)' — the oxygen sensor after your catalytic converter is telling the computer the converter isn't cleaning the exhaust well enough. It's the single most-searched check engine code, and it is not always the converter's fault. A failing downstream oxygen sensor, an exhaust leak before the converter, or an engine running rich can all trigger it. Have those ruled out first: a sensor is around $150–$500, while a converter can be $2,400.
It depends on the code, and on whether the light is steady or flashing. A steady light for an emissions code like P0442 or P0455 is not urgent — you can drive normally and book it in. A flashing check engine light means an active misfire is dumping raw fuel into the catalytic converter, which can destroy it in minutes. If the light is flashing, pull over safely and stop driving. Codes involving oil pressure (P0521) or crankshaft position (P0335) should also be treated as stop-driving situations.
Sometimes. If the fault was intermittent — a loose fuel cap causing P0455, for example — the light usually clears on its own after the computer completes several successful drive cycles, typically within 50–100 miles of mixed driving. If the fault is still present the light will stay on. Disconnecting the battery clears the code but not the cause, and it also wipes the emissions readiness monitors, which means you'll fail an emissions test until you've driven enough for them to reset.
You need something to read the port, but it doesn't have to be expensive. Every car sold in the US since 1996 has a standard OBD-II port under the dashboard on the driver's side. Basic readers cost $20–$40, and Bluetooth dongles that pair with a phone app cost about the same. Many auto parts stores will also read codes for free. Bear in mind that cheap readers only pull generic powertrain codes — transmission, ABS and manufacturer-specific codes usually need a better scanner, which is one reason a code like P0700 needs a proper scan to be useful.
Most independent shops charge $90–$150 for diagnosis, which is typically one hour of labor. That fee buys the technician's time to test and confirm the cause, not just read the code — reading the code is the easy part. Many shops credit the diagnostic fee back against the repair if you have them do the work, so it's worth asking upfront. Be wary of anyone quoting a repair from the code alone without testing.
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Enter your current mileage and driving conditions to see what's due now, what's coming up, and what may already be overdue.
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Enter your car's value, the repair quote, age and mileage to get a clear verdict on whether the repair is worth paying for.
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