Diagnostics, Troubleshooting and Repair Practice
This domain tests systematic fault finding rather than parts replacement. Questions cover symptom verification, scan-tool use, diagnostic trouble codes, electrical and mechanical tests, road testing and confirmation of repair.
Diagnostic method
- Verify the customer complaint, then gather information: when it occurs, hot or cold, load, speed and any recent work.
- Check for technical service bulletins and known faults before testing. Connect a scan tool, record trouble codes and freeze-frame data, then clear nothing until the data is saved.
- Test the affected system logically: power, ground, fuses, relays, sensor signals and actuators. Use voltage-drop tests under load rather than resistance alone.
- Mechanical checks include compression, cylinder leakage, vacuum and fuel pressure. An engine needs correct air, fuel, compression and spark to run.
- After repair, clear codes, road test and verify that the original symptom is gone and no new codes return.
Worked example
A scan tool shows P0301, misfire in cylinder 1. Reasoning: swap the ignition coil from cylinder 1 to cylinder 2. If the misfire moves to cylinder 2, the coil is faulty. If it stays on cylinder 1, test the injector, spark plug and compression. This controlled swap avoids buying unnecessary parts.
Traps
The “parts cannon” approach, clearing codes before recording freeze-frame data, ignoring poor earth connections, and failing to verify the repair are common mark-losers. A code identifies a circuit or system, not always the exact failed component. Always confirm with a test.