Vehicle Inspection, Diagnosis and Fault Finding
Domain 7: Vehicle Inspection, Diagnosis and Fault Finding
This domain tests a structured approach to complaints, inspections, diagnostic tools and verification. The goal is to find the root cause, not to replace parts until the symptom disappears.
Core rules
- Verify the customer's complaint and conditions: when, how often, temperature, speed, load and warning lamps.
- Start with a visual inspection and basic checks: fluids, leaks, tyres, battery, fuses, connectors and obvious damage.
- Use a scan tool to read DTCs and freeze-frame data, but treat codes as clues. A code names a circuit or system, not always the failed component.
- Check live data and compare with expected values. Use pinpoint tests for power, ground, signal and mechanical condition.
- Road test safely and legally. Reproduce the symptom under controlled conditions.
- After repair, clear codes only after the repair is verified, then re-test and confirm no return.
- Record the complaint, tests, readings, repair and outcome.
Worked example
A scan tool shows P0171, System too lean bank 1. Long-term fuel trim is +25%. Reasoning: the engine is adding fuel because it detects excess air or insufficient fuel. Check for vacuum leaks first, especially intake boots and hoses; then inspect the MAF sensor, fuel pressure and injector operation. Do not replace the oxygen sensor simply because the code mentions fuel trim; the sensor may be reporting a real lean condition correctly.
Traps
- Replacing parts based on a code alone.
- Clearing codes before recording freeze-frame data.
- Ignoring mechanical faults such as low compression or exhaust restriction.
- Using an unsafe road test.
- Failing to verify the repair under the original conditions.