On-Board Diagnostics: Introduction
The ECM has an on-board diagnostic system, which detects malfunctions related to engine sensors or actuators. See Fig 1. The ECM also records various emission-related diagnostic information. See Fig 2. The Malfunction Indicator Light (MIL) on the instrument panel illuminates when the same malfunction is detected in 2 consecutive trips (2 trip detection logic), or when the ECM enters fail-safe mode.
The MIL is located on the instrument panel. The MIL will light up when the ignition switch is turned to ON position without the engine running. This is a bulb check. If the MIL does not illuminate, see WARNING LIGHTS in appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. When the engine is started, the MIL should go off. If the MIL remains illuminated, the on-board diagnostic system has detected an engine system malfunction.