Diagnosis and Rectification of Vehicle System Faults
Diagnosis and Rectification of Vehicle System Faults
This domain tests systematic fault finding rather than guesswork. You must verify the concern, gather evidence, isolate the cause, repair it and prove the repair. Questions often cover scan-tool data, wiring diagrams, oscilloscope use, road testing and customer interview skills.
The diagnostic process
- Verify the customer concern and conditions when it occurs.
- Gather evidence: freeze-frame data, live data, service history, technical bulletins and wiring diagrams.
- Test to isolate the fault, not merely to read a code.
- Analyse results against expected values.
- Repair the root cause.
- Verify with a road test and re-scan.
A fault code names a circuit or system; it does not always identify the failed component. Check powers, grounds and communication before condemning a module. Voltage-drop testing under load is more reliable than an ohmmeter for high-current circuits. Back-probe with adapters rather than piercing insulation, and compare live data with a known-good vehicle where possible.
Worked example
Intermittent no-crank. Reasoning: check battery state of charge and cranking voltage first; then the starter control circuit; then voltage drop on the engine earth; then immobiliser key authentication; then the neutral or clutch switch. If the main earth voltage drop exceeds about 0.5 V, repair the cable. Only then test the starter. This sequence avoids fitting a starter that was never faulty.
Traps
- Replacing parts because a code points at them.
- Clearing codes and returning the vehicle without verification.
- Ignoring technical service bulletins and known patterns.
- Failing to record the road-test result and final scan.