Automotive Diagnostics and Fault Finding
What this domain tests
Diagnostics tests a logical process: confirming the complaint, gathering evidence, forming a hypothesis, testing it and verifying the repair - not guessing at parts.
Core rules
- The five-step routine: verify the complaint, collect information (customer interview, DTCs, freeze-frame, visual check), analyse, test the hypothesis, repair and confirm.
- OBD-II codes identify a circuit or system, not necessarily the failed component: P0300 means random or multiple misfire, not replace coil.
- Use the scan tool for live data and the multimeter for circuit checks; a code reader alone is not a diagnostic tool.
- Check the basics first: battery voltage, earths, fuses, vacuum hoses, fluid levels and obvious damage.
- Compare actual values with specification; a sensor reading outside range may be wiring, not the sensor.
- After repair, clear codes, run a road test and confirm monitors are ready.
Worked example
A car idles roughly with P0300 and no single-cylinder code. A compression test gives 8 bar dry on all cylinders, rising to 11 bar when a little oil is added. The reasoning: a large rise after oil points to worn piston rings or bores, because the oil seals the gap temporarily; if compression stayed low, leaking valves or a head gasket would be more likely. Spark and fuel are checked next, but the compression evidence directs the diagnosis.
Traps that cost marks
Treating the fault code as the diagnosis, replacing parts without a test, ignoring freeze-frame data, and clearing codes before recording them.