One part of the PCM is devoted to monitoring both input and output functions within the system. This ability forms the core of the self-diagnostic system. If a problem is detected within a circuit, the control module will recognize the fault, assign it an diagnostic trouble code (DTC), and store the code in the memory. The stored codes may be retrieved during diagnosis.
While the electronic engine control system is capable of recognizing many internal faults, certain faults will not be recognized. Because the control module sees only electrical signals, it cannot sense or react to mechanical or vacuum faults affecting engine operation. Some of these faults may affect another component, which will set a code. For example, the PCM monitors the output signal to the fuel injectors, but cannot detect a partially clogged injector. As long as the output driver responds correctly, the computer will read the system as functioning correctly. However, the improper flow of fuel may result in a lean mixture. This would, in turn, be detected by the oxygen sensor and noticed as a constantly lean signal by the PCM. Once the signal falls outside the pre-programmed limits, the control module would notice the fault and set a trouble code.MALFUNCTION INDICATOR LAMP (MIL)
The malfunction indicator lamp (MIL) is located on the instrument panel. The lamp is labeled "Check Engine" on 1993-97 vehicles, and "Service Engine Soon" on 1998-00 vehicles. The lamp is connected to the PCM, and will alert the driver to certain malfunctions within the electronic engine control system. When the lamp is illuminated, the PCM has detected a fault and stored a DTC in the memory.
The light will stay illuminated as long as the fault is present. Should the fault self-correct, the MIL will extinguish, but the stored code will remain in the memory.
Under normal operating conditions, the MIL should illuminate briefly when the ignition key is turned ON . This is an MIL circuit check to verify that the lamp operates. If the lamp does not light, the bulb and/or circuit should be repaired. As soon as the PCM receives a signal that the engine is cranking, the lamp should extinguish. The lamp should remain extinguished during the normal operating cycle.
The MIL may also be used for retrieving DTCs when the PCM is put in diagnostic mode. When in diagnostic mode, the MIL will emit a series of flashes, indicating which DTCs are stored in the PCM memory.