On-Board Diagnostics: Notes

NOTE: Before performing any testing procedures, check for any related technical service bulletins.

NOTE: Malfunction Indicator Light (MIL) may also be referred to as the SERVICE ENGINE SOON light.

NOTE: For more in-depth information on engine performance related systems and components, see appropriate THEORY & OPERATION article.

MIL is either located in the message center on top of driver's side of instrument panel or on the right side of the instrument cluster. MIL is displayed as an engine icon. When ignition is first turned on, MIL should come to verify bulb and circuit operation.

The Powertrain Control Module (PCM) monitors several different engine control system circuits. If malfunction occurs which affects vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM and PCM will enter limp-in mode. In limp-in mode, PCM substitutes values for the failed component and provides continuous engine operation. When DTC is stored in PCM, the PCM will turn on MIL and MIL will remain on steady. If problem exists which does not affect vehicle emissions, a DTC will be stored in PCM but MIL will not be turned on.

PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the criteria for DTC has not been met. For example, assume criteria for DTC requires PCM to monitor a specified sensor output circuit only when engine operates at 750-2000 RPM. If the sensor output circuit becomes shorted to ground when engine is operating at 2400 RPM, DTC will not be stored, as short to ground occurred at 2400 RPM which exceeds the limit when PCM monitors the circuit.

If PCM detects active engine misfire severe enough to cause catalytic converter damage, MIL will flash to warn the driver of possible damage and DTC will be stored.

The PCM will turn off MIL when malfunction is not detected during 3 consecutive trips on all applications except an engine misfire monitor or fuel system monitor. For additional information on engine misfire monitor and fuel system monitor, see MONITORED CIRCUITS . The PCM will turn off MIL when malfunction is not detected during 3 engine misfire or fuel system monitor tests. The PCM performs engine misfire or fuel system monitor tests when engine is operating within 375 RPM and within 10 percent of the load of operating condition at which first malfunction was detected.

DTCs may be retrieved for system diagnosis. See RETRIEVING DIAGNOSTIC TROUBLE CODES . System malfunctions are identified as either hard failures or intermittent failures. For additional information on hard failures or intermittent failures, see HARD FAILURES and INTERMITTENT FAILURES .