System Description

PURPOSE
The On-Board diagnostics incorporated with the Powertrain Control Module (PCM) are intended to assist the field technician in repairing vehicle problems by the quickest means.

The PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "On-Board Diagnosis".

OPERATION
Certain criteria -- arming conditions -- must be met for a trouble code to be set in PCM memory. The criteria may include: engine rpm, engine temperature, and/or PCM input voltage. If a problem is sensed in a monitored circuit, and all of the criteria -- 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 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 arming conditions are met, a trouble code will be set in PCM memory.

Trouble Code Diagnosis
Each trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of trouble codes as well as no trouble code problems. It is not necessary to perform all of the tests to diagnose an individual code.

Hard Codes
A hard code is a code that comes back within one cycle of the ignition key. This means the defect is there every time the PCM checks that circuit or function. Procedures included here verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an intermittent test must be performed.

Intermittent Codes
A diagnostic trouble code that is not there every time the powertrain control module checks the circuit is an "intermittent" code.

Most intermittent codes are caused by wiring or connector problems. Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them.

Try to reproduce the conditions that cause the intermittent condition to occur.

Reset Counter
The reset counter counts the number of times the vehicle has been started since codes were last set, erased, or the battery was disconnected. The reset counter will count up to 255 start counts.

The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on a suitable scan tool.

When there are no trouble codes stored in memory, the scan tool will display "No trouble codes found" and the reset counter will show "Reset Count = XXX".

Always begin diagnosis by checking for trouble codes. This will direct you to the specific test or tests that should be performed. See Diagnostic Charts. Diagnostic Trouble Code Descriptions