On-Board Diagnostics: Notes

NOTE: Before performing any testing procedures, check for any related Technical Service Bulletins (TSBs).

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

MIL is located on instrument cluster, just below the tachometer and is displayed as an engine icon. When ignition is first turned on, MIL should come on and remain on for 2 seconds to verify bulb and circuit operation, and then go off.

NOTE: If MIL does not come on and remain on for 2 seconds and then go off when ignition is first turned on, check instrument cluster for diagnosis of MIL. See ANALOG INSTRUMENT PANELS - DAKOTA & DURANGO article in ACCESSORIES & EQUIPMENT.

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 set in PCM and PCM enters limp-in mode. In limp-in mode, PCM substitutes values for failed component and provides continuous engine operation. When DTC is set in PCM, the PCM will turn on MIL and MIL will remain on steady. In some instances, the MIL is turned on when the failure exists and DTC was set. These instances are referred to as one trip DTC. In may instances, the emission system must fail more then once before the PCM will turn on the MIL. These are referred to as a 2 trip DTC. For additional information for trip definition, see TRIP DEFINITION . If problem exists which does not affect vehicle emissions, a DTC will be set in PCM but MIL will not be turned on. OBD-II related DTCs are entered in PCM according to individual priority. The DTC priority determines when MIL is turned on and DTC self-erasure. For additional information for DTC priority and DTC self-erasure, see DTC PRIORITY & DTC SELF-ERASURE . Non-emission related DTCs have no priority in which they are entered in the PCM.

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 between 750 RPM and 2000 RPM. If the sensor output circuit becomes shorted to ground when engine is operating at 2400 RPM, DTC will not be set, 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 set.

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 .