Vehicle diagnosis and troubleshooting
Vehicle diagnosis and troubleshooting
This domain tests logical fault-finding: verify the complaint, gather data, test, repair and confirm.
Core rules
- Verify the symptom yourself; a rough idle may be a misfire, a mount or an exhaust leak.
- Use DTCs and freeze frame as clues, not verdicts. Record all codes and live data before clearing.
- Fuel trims: positive means the ECU is adding fuel, as with lean or unmetered air; negative means removing fuel, as with rich running. Compare bank 1 and bank 2 on V engines.
- No-start: check battery voltage while cranking, then spark, fuel pressure, injector pulse and compression. Do not replace parts by guesswork.
- Use a scope for ignition, injector and CAN signals; a multimeter averages fast pulses and can mislead.
Worked example
Crank but no start. Battery is 12.4 V static and 10.8 V cranking. Scan shows no rpm signal. Check the crankshaft position sensor and reluctor; a 0 V signal while cranking suggests an open sensor or wiring. If rpm shows on scan but there is no spark, suspect the ignition coil or driver. Compression is checked only after spark and fuel are confirmed.
Traps
- Clearing codes and returning the car, erasing evidence.
- Treating a P0300 random misfire as a single bad coil.
- Using starting fluid on a diesel with glow plugs or an intake heater.
- Replacing a sensor because its resistance is out of specification without checking the reference and ground.