- Description of On-Board Diagnostics

1. Following a Flow of Diagnosis will usually lead to the correct test procedure and help repair the problem.

2. The Diagnostic Circuit Check (under Computers and Control Systems/ Testing Procedures) should ALWAYS be performed whether or not a computer related problem is suspected. The "Diagnostic Circuit Check" may reveal computer problems which were not originally suspected. Bypassing this test during diagnosis may lead to replacement of good parts.

3. If test procedures cannot fix the problem (or is not available), you can find additional component and system information in these areas:

^ A-Charts (System Testing) - may contain useful system descriptions, circuit description, wiring diagrams or diagnostic notes. Look here to see if the system you are working on has a related system test. You may perform these tests even if you have not been directed (unless otherwise noted). Testing and Inspection


^ C-Charts (Component Testing) - may contain useful component descriptions, circuit description, wiring diagrams or diagnostic notes. Look here to see if the component you are working on has a specific component test. You may perform these tests even if you have not been directed (unless otherwise noted). Testing and Inspection

^ Diagnostic Trouble Code Charts - may be reviewed for diagnostic notes and circuit descriptions. Testing and Inspection

CAUTION:
- Trouble Code charts should Never be used to diagnose a component unless that trouble code has been stored.

- Using the trouble code chart without a stored trouble code may lead to incorrect diagnosis and replacement of good parts.

^ Diagnosis by Symptoms - should be used when no computer related problems appear. Refer to the related system (Computers and Control Systems, Fuel Control Systems, Ignition Control System, etc.) Testing and Inspection

^ System or Component Descriptions - Sometimes getting a better idea of HOW the system or component works can help with diagnosis of a problem.


NOTE: Depending on application, some diagnostic charts available for one vehicle may not necessarily be applicable to others (i.e. There is no Chart C-7 for vehicles without EGR systems.)