When your check engine light comes on, your car stores a diagnostic trouble code. Browse our plain-English guides below to understand what your code means and what to do next.
50 codes covered and growing.
Showing 50 of 50 codes
P0420 means the catalytic converter on Bank 1 (driver's side on most vehicles) isn't cleaning exhaust gases efficiently enough. It does not automatically mean the converter needs replacement — faulty oxygen sensors, exhaust leaks, misfires, and rich-running conditions can all trigger P0420 on an otherwise functional converter. Diagnosis comes first.
P0171 means the engine is running lean on Bank 1 — too much air or too little fuel in the mixture. The ECM detected this through oxygen sensor feedback and confirmed it by watching fuel trims climb to their limit. Common causes are vacuum leaks, a dirty MAF sensor, and fuel delivery issues. The cause depends heavily on where the lean condition occurs — at idle, at cruise, or under hard acceleration.
P0300 means one or more cylinders are misfiring randomly — combustion is not completing correctly. A flashing check engine light with P0300 means stop driving immediately: active misfires send raw fuel into the catalytic converter and can destroy it in minutes. With a steady light, limit driving and diagnose promptly. Common causes are worn spark plugs, failed ignition coils, low compression, and vacuum leaks.
P0455 means the EVAP system detected a large leak in the fuel vapor containment system. Despite the word 'large,' this is often caused by something as simple as a loose or improperly seated gas cap. The car will run normally, but it will fail an emissions test. Compare: P0442 is a small leak, P0455 is a large leak, P0456 is a very small leak.
The engine isn't reaching its normal operating temperature, usually because the thermostat is stuck open.
P0442 means a small leak was detected in the EVAP (fuel vapor) system. It's often caused by a loose, worn, or damaged gas cap — but can also involve EVAP hoses, the purge valve, vent valve, charcoal canister, or a fuel tank seal. It's not an immediate breakdown risk, but the car may fail an emissions test.
The catalytic converter on the passenger side of the engine isn't working efficiently enough.
The mass airflow sensor is sending readings to the ECU that are outside the expected range for current driving conditions.
The intake camshaft on the driver's side is advancing further than the ECU commanded it to.
The ECU isn't receiving a proper signal from the vehicle speed sensor.
The transmission control module has detected a fault and is alerting the powertrain control module.
P0401 means insufficient EGR (Exhaust Gas Recirculation) flow. The system isn't routing enough exhaust gas back into the engine as commanded. Common causes include clogged EGR passages or valve, vacuum or control issues, wiring faults, or sensor feedback problems.
Cylinder 1 isn't firing correctly — combustion isn't happening as it should in that specific cylinder.
Cylinder 2 isn't firing correctly — combustion is incomplete or absent in that specific cylinder.
Cylinder 3 is misfiring — that cylinder isn't completing combustion the way it should.
There's a very small leak in the fuel vapor system — even smaller than a P0442.
The engine is running lean on the passenger side — too much air or not enough fuel on Bank 2.
The throttle position sensor is sending a voltage signal that's too low for what the ECU expects.
The intake air temperature sensor is sending a voltage signal that's higher than what the ECU expects, suggesting a cold or open circuit.
P0606 means the Powertrain Control Module (PCM) — the main computer — detected an internal processing fault. This sounds alarming, but do not replace the PCM without exhaustive diagnosis first. Charging system faults, poor grounds, corroded connectors, wiring harness issues, and failed software updates all produce P0606 on a functioning PCM. PCM replacement is expensive, requires programming, and is the correct fix only after everything else is ruled out.
The heater element inside the upstream oxygen sensor on the driver's side isn't working properly.
The heater inside the downstream oxygen sensor (after the catalytic converter) on the driver's side isn't working.
The heater element in the upstream oxygen sensor on the passenger side isn't functioning correctly.
The downstream oxygen sensor on the driver's side is consistently showing high voltage, indicating it may be stuck or the engine is running rich.
The knock sensor on the driver's side isn't communicating properly with the ECU.
The mass airflow sensor is sending a voltage reading below the minimum threshold — the ECU thinks almost no air is entering the engine.
The manifold absolute pressure sensor is sending readings that don't match what the ECU expects for current engine conditions.
The engine coolant temperature sensor circuit voltage is lower than expected — the ECU thinks the engine is extremely hot.
The engine coolant temperature sensor is sending a higher voltage than expected — usually indicating an open circuit or a sensor that has failed to a 'cold' reading.
The throttle position sensor is sending a signal that's electrically valid but doesn't match what the ECU expects for current driving conditions.
The throttle position sensor is sending a higher voltage than the ECU expects, suggesting a wide-open throttle signal even when the pedal isn't pressed.
The upstream oxygen sensor on the driver's side isn't generating a proper signal — the circuit itself is at fault.
The upstream oxygen sensor on the driver's side is consistently reading a low voltage, indicating a lean exhaust condition or a faulty sensor.
The upstream oxygen sensor on Bank 1 is stuck at a high voltage, suggesting a rich exhaust condition or a faulty sensor.
The engine is getting too much fuel or too little air on the driver's side — the air/fuel mixture is running rich.
The ECU detected a fault in the fuel injector circuit for cylinder 1 — the injector may not be firing or the circuit is open.
Cylinder 4 is not completing combustion properly — it's misfiring.
Cylinder 5 is misfiring — combustion is incomplete or absent in that cylinder.
Cylinder 6 is misfiring — combustion is failing in that specific cylinder.
The crankshaft position sensor isn't sending a proper signal — this can prevent the engine from starting or running.
The camshaft position sensor on Bank 1 isn't sending a reliable signal to the ECU.
The Bank 1 catalytic converter isn't reaching full efficiency quickly enough during engine warm-up.
The EVAP fuel vapor system has a general fault — the ECU detected something isn't working correctly in the system as a whole.
The EVAP system isn't purging stored fuel vapors into the engine at the rate the ECU expects.
The EVAP vent valve circuit has a fault — the valve that allows fresh air into the charcoal canister isn't working properly.
The idle control system isn't maintaining the correct idle speed — the engine's idle is unstable or incorrect.
The engine is idling faster than the ECU has commanded — typically caused by a vacuum leak or stuck-open idle bypass.
The ECU detected that the electrical system voltage is lower than it should be — typically from a weak battery or failing alternator.
The torque converter clutch circuit has a fault — the lockup clutch inside the torque converter may not be engaging or releasing correctly.
Shift Solenoid A in the transmission isn't functioning correctly, which can cause incorrect or missed gear shifts.