Description of On-Board Diagnostics

The Powertrain Control Module (PCM) has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "on-board diagnostics."

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, 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 (arming conditions) has 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 (arming conditions) are met, a trouble code will be stored in the PCM memory.