See Figure 1 through 7
The PCM is designed to test its own input and output circuits. The PCM monitors the input and output signals coming from and going to the various sensors. The PCM then compares the in-coming and out-going signals to specific parameters, which are written into the computer's memory at the factory. If a sensor's signal is showing operation of the sensor to be out of the given range, the computer stores a fault code. The fault code is stored in the PCM for eventual display to the person performing the readout procedure. The fault code does not indicate which component is faulty, rather which circuit was perceived as functioning out of the given parameters. Therefore, once a fault code is known, the entire circuit must be checked for problems, rather than simply replacing the main sensor of the circuit.
The fault codes can be displayed by means connecting a Diagnostic Readout Box (DRB III) and reading a numbered display code, which indicates the circuit in question. Some inputs and outputs are checked continuously and others are checked under certain conditions.
If the problem is repaired or no longer exists, the PCM cancels the fault code after 50-100 key ON/OFF cycles.
When a fault code is detected, it appears as either a flash of the "MIL'' on the instrument panel or by watching the Diagnostic Readout Box (DRB III). This indicates that an abnormal signal in the system has been recognized by the PCM.