20 Most Common Problems Behind a Check Engine Light

P0300 indicates that at least two engine cylinders are misfiring, which can seriously damage the engine or catalytic converter. Cylinder misfires, when an insufficient amount of fuel is burned in a cylinder, are often due to worn-out spark plugs or spark plug wires or a faulty ignition coil.

P0301 is similar to P0300, but indicates a misfire in cylinder No. 1 rather than in two or more cylinders. Both issues cause the engine to run rough and operate with lower power, and can cause serious and expensive damage if left unrepaired for too long.

P0302 indicates that the cylinder No. 2 is misfiring. You risk serious and costly engine damage by ignoring this problem.

P0303 indicates a misfire in the cylinder No. 3. Any misfiring causes serious and expensive damage if left unrepaired for too long.

P0304 indicates a misfiring in the cylinder No. 4. An unrepaired cylinder misfire will eventually lead to costly engine failure.

P0305 indicates the cylinder No. 5 is misfiring. Ignoring any cylinder misfire codes can lead to catastrophic engine damage.

P0306 indicates cylinder No. 6 is misfiring. Cylinder misfire can cause serious engine damage.

P0420 points to the catalytic converter — the emissions-control device(s) located near the exhaust manifold. Catalytic converters typically do not malfunction unless other problems, like a failed spark plug or exhaust valve, have gone unfixed. The code could also indicate other issues, including damaged oxygen sensors, leaking fuel injectors, or cylinder misfires.

P0430 is similar to P0420, in that both focus on issues with the catalytic converter, an emissions control device. The difference is that P0430 identifies an issue with one of the oxygen sensors on the device. If the problem is a faulty oxygen sensor, then the repair job costs less than replacing the entire catalytic converter.

P0171 indicates a problem with the vehicle’s ability to maintain an optimal fuel-to-air ratio, which could mean either too little fuel or too much air. This can cause a lack of power to the engine, engine misfiring or “coughing,” or a rough idle.

P0455 signals a large leak in the evaporative emissions control systems (EVAP), which prevent fuel vapors from being released into the atmosphere by storing them in a canister of charcoal pellets and then sending them into the engine air intake manifold using a purge control valve. This problem could also be caused by a faulty gas cap.

P0456 is similar to P0455, but indicates a small fuel vapor leak (0.20″ in diameter or less) rather than a large one in the evaporative emission control systems (EVAP).

P0442 is similar to P0455 and P0456, indicating a fuel-vapor leak in the evaporative emission control (EVAP) system. P0442 appears when your vehicle has a leak measuring between .020″ to .040″ in diameter.

This is another code indicating a problem with the evaporative emission control (EVAP) system.

P0128 suggests your vehicle is not reaching or maintaining its ideal operating temperature. This problem is minor, but can adversely impact fuel economy and cause higher levels of harmful emissions.

P0174 indicates a fuel-air ratio problem in one of the banks of cylinders. Improper fuel-air ratios can eventually cause serious engine damage. Symptoms include lack of power, higher fuel consumption, and a hesitation/surge in acceleration.

P1000 is a manufacturer-specific trouble code, used mostly by Ford, Jaguar, and Mazda, though some other brands also utilize it. It indicates that the vehicle’s engine control computer did not complete a self-diagnostic cycle related to emissions control. The code doesn’t indicate why the cycle wasn’t completed. This is a minor issue.

P0113 indicates a problem with the intake air temperature (IAT) sensor that regulates the correct amount of spark and fuel injected into the cylinders based on outdoor temperatures. A common problem is simply loose wiring, but it could also indicate a faulty IAT sensor.

P0446 indicates that the vehicle’s evaporative emission control (EVAP) system is not working properly due to a short circuit in the control of the vent control valve. This causes a failure to maintain proper pressure that can lead to a decrease in fuel economy and a fuel smell.

P0102 indicates a problem with the engine’s air intake related to low voltage output from air flow controllers, reducing engine performance. Symptoms include a rough running engine, black smoke from the tailpipe, or stalling. Inadequate air intake over the long-term can eventually lead to more serious engine trouble down the road.