Engine systems
Many check engine codes come from a handful of self-tests. Start with the system that set your code to see how the test works and what to check next.
Pick a system.
Engine misfire codes
A misfire code means the engine computer saw one or more cylinders fail to burn their fuel properly. P0300 means random or multiple cylinders. P0301 through P0308 name the cylinder.
02 / EVAPEVAP system codes
The evaporative emissions (EVAP) system keeps fuel vapor from escaping the tank. It stores the vapor in a charcoal canister and later draws it into the engine to burn. The car tests that system for leaks, and a failed test turns on the check engine light.
03 / CATALYSTCatalytic converter codes
A catalyst code such as P0420 means the computer judged the catalytic converter’s efficiency below its limit. The monitor uses sensors before and after the converter, so a sensor, an exhaust leak or an engine problem can affect the result as well as the converter itself.
04 / FUEL TRIMFuel trim and lean or rich codes
Fuel trim is the percentage the engine computer adds to or takes away from its base fuel amount to keep the air-fuel mixture on target. A positive number means it is adding fuel because the engine was running lean. A negative number means it is taking fuel away.
Cooling, transmission and network guides are planned. For now, those codes have their own pages in the code library.