On-Board Diagnostics: Notes

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

The PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called on-board diagnosis. Certain criteria, or arming conditions, must be met for a trouble code 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 trouble code will be stored in the PCM. It is possible that a trouble code 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 have not 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. The On-Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBD-II/Euro Stage III OBD. These OBD-II/Euro Stage III OBD Diagnostics control the functions necessary to meet the requirements of California OBD-II, Federal OBD regulation and European regulation. These requirements specify the inclusion of a Malfunction Indicator Light (MIL) located on the instrument panel. The purpose of the MIL is to inform the vehicle operator in the event of a malfunction of any emission system or component. MIL may also be referred to as CHECK ENGINE light. MIL is located on left side of instrument cluster and is represented by an engine symbol. When ignition is turned on, MIL should come on to verify bulb and circuit operation. If MIL does not turn on, diagnose MIL indicator. See INSTRUMENT CLUSTER under SYSTEM TESTS in appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT.

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, 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.

DTCs may only be retrieved for system diagnosis by using a scan tool. 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 .