Diagnostic Strategies

Note: For catalytic converter and feedback system to function properly, it is essential that vehicle is at operating temperature, (except when attempting to diagnose cold start or cold driveability problem).

1) Verify Problem
Use the Vehicle Repair History And Driveability Worksheets In Technical/ Management Support to provide better communication between the customer and the mechanic.

- The vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrect components or unapproved repairs can have subtle influences on seemingly unrelated systems.

Always verify the customer complaint. You must know the problem if you are to diagnose it.

Check any Technical Service Bulletins that may apply to the vehicle being serviced.

2) Extract And Clear Fault Codes
- Diagnostic Trouble Codes are cleared whenever the Engine Controller or the battery is disconnected. Therefore, all fault codes should be read prior to any vehicle power interruption or troubleshooting.

- Diagnose and repair any fault code related problems before proceeding with further diagnosis.

Note: This process also verifies PCM and control system operation.

3) Perform Visual Inspection:
- PCM grounds: clean, tight, and in their proper location.
- Vacuum hoses: splits, kinks and proper connections. Check thoroughly for any type of leak or restriction.
- Air leaks at throttle body mounting area and intake manifold sealing surfaces.
- Ignition wires for cracking, harness, proper routing, arcing and carbon tracking.
- Wiring for proper connections, pinches, and cuts.
- Harnesses (EST, ESC, etc...) for proper routing.

4) Check and Record Idle RPM

Warning: Be sure to apply parking brake and/or block wheels before performing any test with the engine running.

If vehicle is equipped with an automatic transaxle, test and record idle rpm when the vehicle is first brought into shop, to assist in diagnosing;

- Engine Stalling
- Creeping
- Hard Shifting

SEE Computers and Control Systems/ Testing And Inspection/ Procedures/ Component Tests/ Throttle Body Minimum Airflow Test.