The engine controller monitors critical input and output circuits, making sure they are operational. Some circuits are checked continuously, and some are checked only under certain conditions.
Integrated into the engine controller is the On-Board Diagnostic (OBD) system. If the OBD system detects a bad circuit, it will store a DTC into memory. Each fault detected is assigned a specific code (see listing of DTCs). The code will stay as long as the circuit is perceived to have a problem. If the problem does not occur again, the memory will clear after fifty engine start-ups.
The DTC's are read via the malfunction indicator lamp (CHECK ENGINE lamp). A DTC description can also be provided by the professional DRBII scan tool, or equivalent.
A DTC is a general guide to point the diagnosing technician in the right direction of a problem in the engine management system. The OBD system monitors whole circuits, not individual components within the circuits. Thus, a DTC does not identify which component of a given circuit has failed and so it is to be treated as a symptom that may or may not be a real problem. In some cases, because of the design of the diagnostic test procedure, a DTC can be the reason for another DTC to be set. Thus it is vital that a systematic (process-of-elimination) troubleshooting procedure be followed.