Browse common check engine codes and start a free diagnosis.
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means your engine computer has detected that the catalytic converter on Bank 1 is not cleaning exhaust gases as efficiently as it should. It is one of the most common — and most misunderstood — check engine codes.
System Too Lean (Bank 1)
P0171 means the engine computer is adding extra fuel because the air-fuel mixture on Bank 1 is running lean — too much air, not enough fuel. Left unfixed, it can cause rough idle, hesitation, and long-term engine damage.
Random/Multiple Cylinder Misfire Detected
P0300 means the engine computer has detected misfires across multiple cylinders. A misfire happens when fuel fails to ignite properly in the combustion chamber. The last digit(s) in related codes (P0301, P0302, etc.) point to a specific cylinder.
Evaporative Emission System Leak Detected (Large)
P0455 means the evaporative emissions (EVAP) system has detected a large leak. The EVAP system captures fuel vapors from the gas tank so they don't escape into the atmosphere. A large leak often points to a loose or missing gas cap.
Coolant Thermostat Below Regulating Temperature
P0128 means the engine is not reaching its proper operating temperature. The engine computer expects the coolant to reach a set temperature within a certain time; if it doesn't, the thermostat may be stuck open.
Evaporative Emission System Leak Detected (Small)
P0442 means the EVAP system has detected a small leak — smaller than the large leak of P0455 but still enough to let fuel vapors escape. Small leaks are harder to find and often require a smoke test.
Mass Air Flow Circuit Low Input
P0102 means the mass air flow (MAF) sensor is reporting less air entering the engine than the computer expects. This can cause rough running, poor fuel economy, and stalling.
Idle Air Control System RPM Higher Than Expected
P0507 means the engine computer has detected the idle speed is higher than the target. The idle air control system (or electronic throttle body on modern cars) regulates idle RPM; a higher-than-commanded idle points to a vacuum leak or idle control issue.
Camshaft Position Timing Over-Advanced (Bank 1)
P0011 means the engine computer has detected that the camshaft timing on Bank 1 is more advanced than commanded. This relates to the variable valve timing (VVT) system, which adjusts cam timing for performance and efficiency.
Turbocharger / Supercharger Underboost Condition
P0299 means the turbocharger or supercharger is not producing the boost pressure the engine computer expects. The engine will run with reduced power (limp mode on some vehicles).
Crankshaft Position Sensor Circuit Malfunction
P0335 means the engine computer is not getting a proper signal from the crankshaft position sensor. This sensor tells the computer the crank's position so it can time fuel injection and spark. Without it, the engine may not start or may stall.
Exhaust Gas Recirculation Flow Insufficient Detected
P0401 means the exhaust gas recirculation (EGR) system is not flowing enough exhaust gas back into the engine. The EGR system reduces combustion temperatures and emissions; insufficient flow can cause pinging, higher emissions, and check engine codes.