On-Board Diagnostics: Notes

NOTE: For more in-depth information on engine performance related systems and components, see THEORY & OPERATION - CONCORDE, INTREPID & 300M article.

NOTE: The Powertrain Control Module (PCM) and the Transmission Control Module (TCM) are incorporated into one control module. The module is the Next Generation Controller (NGC) and will be referred to throughout this article as the PCM.

NOTE: If the MIL does not come on when turning the key from off to on, bulb may be defective, or a problem may exist on the Programmable Communication Interface (PCI) bus circuit between PCM, BCM and instrument cluster. Check instrument cluster for inoperative MIL. Perform instrument cluster self-diagnostics. See SELF-DIAGNOSTIC SYSTEM in appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. MIL may also be referred to as CHECK ENGINE light.

The PCM has been programmed to monitor several different engine control system circuits. PCM is located on left side of engine compartment. See Figure. Certain criteria, or arming conditions, must be met for a Diagnostic Trouble Code (DTC) to be entered into the PCM memory. The criteria may be a range of engine RPM, engine temperature, and/or input voltage to the PCM. If a problem is sensed with a monitored circuit, and all of the criteria or arming conditions are met, then a DTC will be stored in the PCM. It is possible that a DTC for a monitored circuit may not be entered into the PCM memory even though a malfunction has occurred. This may happen because one of the trouble code criteria may not have been met. The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria are met, a trouble code will be stored in the PCM memory.

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.

If PCM detects an active engine misfire severe enough to cause catalytic converter damage, MIL will immediately start flashing to warn driver of possible catalytic converter damage and DTC will be stored. PCM may turn off MIL if malfunction is not detected during 3 consecutive trips or 3 consecutive engine misfire monitor tests or fuel system monitor tests. For additional information on good trips, see TRIP INDICATOR . For additional information on engine misfire monitor and fuel system monitor, see MONITORED CIRCUITS . PCM performs engine misfire and fuel system monitor tests within predetermined engine speed (RPM) and load operating conditions.

To retrieve DTCs, 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 or INTERMITTENT FAILURES .