On-Board Diagnostics: Notes

The Malfunction Indicator Light (MIL) is located on instrument panel and is displayed as CHECK ENGINE light. The MIL is controlled by the Engine Control Module (ECM) and is used to indicate that the ECM has detected a problem that affects vehicle emissions and a Diagnostic Trouble Code (DTC) is present. When ignition is first turned on, MIL should come on and remain on until engine is started to verify bulb and circuit operation. If MIL does not illuminate at all, diagnose MIL circuits. See MALFUNCTION INDICATOR LIGHT INOPERATIVE  under SYSTEM TESTS. The ECM is programmed with test routines that check the operation of the various systems that the ECM controls. Some tests check the internal functions of the ECM. Some tests are run continuously while other tests run only under specific conditions (referred to as Conditions For Running DTC). When vehicle is operating within conditions for running a particular test, the ECM checks certain parameters and determines whether or not the values are within an expected range. When conditions for setting the DTC are met, the ECM will store a DTC and, if code is emission related, illuminate the MIL. Some DTCs alert the driver via a message. See CODE TYPES . The ECM also saves data and input parameters when most DTCs are set. This data is stored in FREEZE FRAME and/or FAILURE RECORDS. See FREEZE FRAME  and FAILURE RECORDS  .