On Board Diagnostics II (OBD II): Diagnostic Trouble Codes

Diagnostic Trouble Codes (DTCs) used in OBD II vehicles will begin with a letter and are followed by four numbers. The letter at the beginning of the DTC identifies the function of the monitored device that has failed.
P - Powertrain device
C - Chassis device
B - Body device
U - Network or data link code.

The first number indicates if the code is generic (common to all manufacturers), or if it is manufacturer specific. A "0" indicates generic, "1" indicates manufacturer-specific.

The second number indicates the system that is affected with a number between 1-7. The following is a list showing what numbers are assigned to each system.
1 - Fuel and air metering
2 - Fuel and air metering (injector circuit malfunctions only)
3 - Ignition system or misfire
4 - Auxiliary emission controls
5 - Vehicle speed controls and idle control system
6 - Computer output circuits
7 - Transmission

The last two numbers of the DTC indicate the component or section of the system where the fault is located.