Vehicle Diagnosis, Servicing and Preventive Maintenance
Diagnosis and servicing
The exam tests structured diagnosis, periodic service and preventive maintenance. Use the five-step method: verify the complaint, gather information, test, repair, and confirm. A scan tool gives DTCs, freeze-frame and live data; it does not replace mechanical tests. Interview the customer about when the fault occurs, road-test with permission, and check service history. For a lean code, inspect vacuum hoses, intake boots, MAF, fuel pressure and exhaust leaks. Record measurements on the job card. Preventive maintenance follows the maker’s schedule: engine oil and filter, air filter, brake inspection, coolant, tyres, lights, battery and belts.
Worked example
A car sets P0171, lean bank 1. Freeze frame shows idle speed 750 rpm, MAF 2.8 g/s, long-term trim +24%. Reason: the ECU is adding fuel because unmetered air or low fuel delivery is present. Spray carburettor cleaner around intake joints; if trim changes, find the vacuum leak. If not, check fuel pressure and MAF. Replace parts only after the test proves the cause.
Traps
Do not clear a DTC and return the car without repair; the code will reset and the monitor may not be ready. Do not replace a sensor because its code appears; check wiring, connectors and mechanical causes. Skipping the road test or the service reset costs marks.