On-Board Diagnostics

The Powertrain Control Module (PCM) has been programmed to monitor many different circuits of the fuel injection, ignition, emission and engine systems. This monitoring is called "on-board diagnosis." If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the PCM's memory. If the code applies to a non-emissions related component or system, and the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles. Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator (check engine) Lamp. Refer to Malfunction Indicator Lamp.

Certain criteria, or "arming conditions," must be met before a trouble code is 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.

The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 - 2000 rpm. Suppose the sensor's output circuit shorts to ground when engine operates above 2400 rpm (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the PCM will not store a DTC.

The PCM compares input signal voltages from each input device with the 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 (arming conditions) are met, a trouble code will be stored in the PCM memory. For more information on the operating conditions for which the PCM monitors and sets DTC's, refer to Monitored Systems, Components, and Non-Monitored Circuits.

Technicians must retrieve stored DTC's by connecting the DRB III Scan Tool (or an equivalent scan tool) to the 16-way data link connector.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, connect the DRB III scan tool to the 16-way data link connector to erase all DTC's and extinguish the MIL (Check Engine Lamp).

The On-Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBD II. These OBD II Diagnostics control the functions necessary to meet the requirements of California OBD II and Federal OBD regulation. These requirements specify the inclusion of a Malfunction Indicator Light (MIL) located on the instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of the MIL is to inform the vehicle operator in the event of a malfunction of any emission system or component.

OBD II Monitor Information:

The above table summarizes the operation of the various OBD II monitors.