Diagnosis, troubleshooting and repair procedures
Domain focus
This domain covers diagnosis, troubleshooting and repair procedures. The exam checks systematic complaint verification, use of scan data and basic tests, root-cause reasoning, repair quality and confirmation that the fault is fixed.
Core rules
- Verify the complaint, gather customer and service history, then inspect before testing; do not start by replacing parts.
- Use a logical sequence: symptom, possible causes, test to isolate, repair, verify. A DTC points to a circuit or system, not automatically to a component.
- Use live data, voltage-drop tests, compression or leak-down tests, fuel-pressure and vacuum tests as appropriate.
- Check connectors, grounds, fuses and wiring before condemning a sensor or module.
- After repair, clear codes only after recording them, perform the required relearn or adaptation, and road-test or recheck the original symptom.
Worked example
A car cranks but does not start. The technician confirms strong cranking, then checks for spark and injector pulse. No spark and no pulse suggest a crankshaft position signal or immobiliser issue, so the next test is signal presence at the ECU while cranking. If the signal is missing, inspect the sensor, reluctor and wiring. Replacing the ignition coil first would be guesswork because the coil is not proven faulty.
Traps
Clearing codes before recording freeze-frame data; treating a DTC as a parts order; ignoring poor grounds; skipping the post-repair verification; diagnosing a mechanical noise as electrical without a mechanical check.