Diagnostics, Troubleshooting and Vehicle Inspection
Diagnostics, Troubleshooting and Vehicle Inspection
This domain tests a systematic diagnostic process, scan-tool use, DTC interpretation, road testing and inspection. Expect five-step diagnosis, freeze frame, fuel trims, oscilloscope use, compression and leak-down tests, brake and tyre inspection, and roadworthiness checks.
Core rules and concepts
- Verify the complaint, gather information, check for technical service bulletins and do a visual inspection before testing.
- Scan all modules and record DTCs and freeze-frame data before clearing anything.
- Do not replace parts without testing. Use a wiring diagram and pinpoint tests.
- Mechanical tests include compression, leak-down and vacuum. Electrical tests include voltage drop and current ramp.
- Road-test safely and reproduce the customer's conditions where possible.
- Inspection covers tyre tread and damage, brakes, lights, steering play, suspension, emissions, chassis rust and fluid leaks.
- Verify the repair and confirm readiness monitors before returning the vehicle.
Worked example
A car sets P0171 lean bank 1. Long-term fuel trim is +18% at idle and +8% at 2500 rpm. Reasoning: the lean condition is worse at idle, when manifold vacuum is highest, so a vacuum leak is more likely than a fuel-pressure fault. Check intake hoses and gaskets, repair the leak, clear codes only after the repair and verify trims.
Traps that cost marks
Clearing codes before recording freeze frame; replacing sensors by guesswork; ignoring pending codes; skipping basic checks; passing a tyre with cords exposed; and failing to document the repair.
Key points
- Record freeze frame before clearing codes.
- Test before replacing parts.
- Fuel-trim behaviour helps separate vacuum leaks from fuel faults.
- Inspection includes tyres, brakes, lights, steering and structure.
- Verify the repair and monitor readiness.