On-Board Diagnostic System

PURPOSE
The Powertrain Control Module (PCM) has been programmed to monitor many different circuits of the fuel injection system.

OPERATION
If a problem is sensed in a monitored circuit often enough to indicate an actual problem, a diagnostic trouble code (DTC) is stored. The DTC will be stored in the PCM memory for eventual display to the service technician. If the problem is repaired or ceases to exist, the PCM cancels the DTC after 51 engine starts.

Certain criteria must be met for a DTC to be entered into PCM memory. The criteria may be a specific range of engine RPM, engine temperature and/or input voltage to the PCM.

It is possible that a DTC for a monitored circuit may not be entered into memory even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. Example: assume that one of the criteria for the MAP sensor circuit is that the engine must be operating between 750 and 2000 RPM to be monitored for a DTC. If the MAP sensor output circuit shorts to ground when the engine RPM is above 2400 RPM, a 0 volt input will be seen by the PCM. A DTC will not be entered into memory because the condition does not occur within the specified RPM range.

A DTC indicates that the PCM has recognized an abnormal signal in a circuit or the system. A DTC may indicate the result of a failure, but never identify the failed component directly.

There are several operating conditions that the PCM does not monitor and set a DTC for. Refer to the following for Monitored Circuits and Non-Monitored Circuits.

MONITORED CIRCUITS
The PCM can detect certain problems in the fuel injection system.

Open or Shorted Circuit - The PCM can determine if sensor output (which is the input to PCM) is within proper range. It also determines if the circuit is open or shorted.

Output Device Current Flow - The PCM senses whether the output devices are hooked up. If there is a problem with the circuit, the PCM senses whether the circuit is open, shorted to ground or shorted to (+) voltage.

Oxygen Sensor - The PCM can determine if the oxygen sensor is switching between rich and lean. This is, once the system has entered Closed Loop.

NON-MONITORED CIRCUITS
The PCM does not monitor the following circuits, systems or conditions that could have malfunctions that result in driveability problems. A DTC may not be displayed for these conditions.

Fuel Pressure - Fuel pressure is controlled by the fuel pressure regulator. The PCM cannot detect a clogged fuel pump inlet filter, clogged in-line fuel filter, or a pinched fuel supply or return line. However, these could result in a rich or lean condition causing an oxygen sensor DTC to be stored in the PCM.

Secondary Ignition Circuit - The PCM cannot detect an inoperative ignition coil, fouled or worn spark plugs, ignition cross firing, or open circuited spark plug cables.

Engine Timing - The PCM cannot detect an incorrectly indexed timing chain, camshaft sprocket or crankshaft sprocket. The PCM also cannot detect an incorrectly indexed distributor. However, these could result in a rich or lean condition causing an oxygen sensor DTC to be stored in the PCM.

Cylinder Compression - The PCM cannot detect uneven, low, or high engine cylinder compression.

Exhaust System - The PCM cannot detect a plugged, restricted or leaking exhaust system.

Fuel Injector Malfunctions - The PCM cannot determine if the fuel injector is clogged, or the wrong injector is installed. However, these could result in a rich or lean condition causing an oxygen sensor DTC to be stored in the PCM.

Excessive Oil Consumption - Although the PCM monitors exhaust stream oxygen content through oxygen sensor (closed loop), it cannot determine excessive oil consumption.

Throttle Body Air Flow - The PCM cannot detect a clogged or restricted air cleaner inlet or air filter element.

Evaporative System - The PCM will not detect a restricted, plugged or loaded EVAP canister.

Vacuum Assist - Leaks or restrictions in the vacuum circuits of vacuum assisted engine control system devices are not monitored by the PCM. However, a vacuum leak at the MAP sensor will be monitored and a diagnostic trouble code (DTC) will be generated by the PCM.

Powertrain Control Module (PCM) System Ground - The PCM cannot determine a poor system ground. However, a DTC may be generated as a result of this condition.

Powertrain Control Module (PCM) Connector Engagement - The PCM cannot determine spread or damaged connector pins. However, a DTC may be generated as a result of this condition.

HIGH AND LOW LIMITS
The PCM compares input signal voltages from each input device. It will establish high and low limits that are programmed into it for that device. If the input voltage is not within specifications and other diagnostic trouble code (DTC) criteria are met, a DTC will be stored in memory. Other DTC criteria might include engine RPM limits or input voltages from other sensors or switches. The other inputs might have to be sensed by the PCM when it senses a high or low input voltage from the control system device in question.