Some common on-board diagnostics, or OBD, engine trouble codes include P0420, P0171, P0401, P0174, P0442 and P0300. These codes correspond to low efficiency in the catalyst system, lean in bank 1 of the fuel trim system, inadequate exhaust gas re-circulation flow, lean in bank 2 of the fuel trim system, tiny leakage in the evaporative emission system and random engine misfire, respectively.
OBD trouble codes are five-character alphanumeric designations that begin with a letter followed by four numbers. Malfunctions in the vehicle's powertrain, body and chassis are identified by the letters "P," "B" and "C," respectively. Problems of unknown origin start with the letter "U." An OBD code that contain a "0" following the letter indicates that it is generic and applicable to all manufactures, while a "1" after the letter means that the code is manufacturer specific.
The third character identifies what system is affected, and the last two numbers specify what section of the system is faulty. For instance, the common OBD code P0302 means a problem due to a cylinder 2 misfire in the ignition system of any vehicle's powertrain. The OBD code P0304 indicate a cylinder 4 misfire in the powertrain's ignition system.
Other common OBD trouble codes include P0455, P0141 and P0325. These codes signify considerable leakage in the evaporative emission system, a malfunctioning oxygen sensor heater in bank 1 of the emissions management system and a problem with the PCM knock sensor circuit respectively.