Reading Diagnostic Trouble Codes

STORED FAULTS
The Powertrain Control Module (PCM) has been programmed to monitor many different circuits of the fuel injection system. If a problem is sensed in a monitored circuit often enough to indicate an actual problem, a fault is stored. It 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 fault after 51 engine starts.

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

OPERATING CONDITIONS
There are several operating conditions that the PCM does not monitor and set faults for. Refer to Monitored Circuits and Non-Monitored Circuits in this section.

EXTRACTING FAULTS
Stored fault codes can be displayed by cycling the ignition key ON - OFF - ON - OFF - ON. They can also be displayed, on the read out of a Diagnostic Scan Tool. The Scan Tool connects to the diagnostic connector in the vehicle, near the PCM.