The computer control module (ECM, PCM or VCM) performs a continual self-diagnosis on many circuits of the engine control system. If a problem or irregularity is detected, the module will set a Diagnostic Trouble Code or DTC in the computer memory. A DTC indicates a suspected failure that currently exists or that has recently existed in a computer monitored system. A currently present code will usually illuminate the CHECK ENGINE or SERVICE ENGINE SOON light (as equipped), also known as the the Malfunction Indicator Lamp (MIL).
Diagnostic codes are stored in the computer module memory until they are manually erased by a scan tool or until power is disconnected from the module for a certain length of time. If a module detected problem is intermittent and disappears, the MIL will normally extinguish after 10 seconds, but the DTC will remain in diagnostic memory. Intermittent problems are often caused by corroded, dirty or loose electrical connections. A thorough inspection of the affected system should always be the first step in trouble shooting a DTC.
The CHECK ENGINE or SERVICE ENGINE SOON MIL will illuminate whenever the ignition is turned ON with the engine not running. This occurs for a bulb circuit check. As long as the light extinguishes after the engine is started, there are no currently detected trouble codes. Do not confuse the bulb check with a computer module detected DTC.