Vehicle Diagnostics
This domain tests systematic fault diagnosis using symptoms, scan tools, live data, freeze-frame, DTCs, oscilloscopes and wiring diagrams. The exam rewards a logical test sequence rather than parts swapping.
Core rules
Start with the customer complaint and confirm the symptom. Check for technical service bulletins and known faults. Connect a scan tool and record DTCs, freeze-frame and readiness monitors before clearing anything. Use live data to compare actual values with expected values. Test the circuit at the component with a load, not just a voltmeter. A code names a circuit or system, not a guaranteed failed part. Fuel trims show how the ECU is correcting mixture: positive trim means the ECU is adding fuel because the engine is lean.
Worked example
A car sets P0171 and P0174, lean on both banks, with long-term fuel trim at +25 percent at idle and +8 percent at 2500 rpm. The trims improve with engine speed, which points to unmetered air entering at idle, such as a split intake boot or leaking manifold gasket, rather than a weak fuel pump. The correct next step is a smoke test and intake inspection, not fitting new oxygen sensors.
Traps
Replacing an oxygen sensor because a lean code is stored; clearing codes before recording freeze-frame; confusing pending and confirmed DTCs; and ignoring fuel-trim behaviour at different loads. A vacuum leak typically affects both banks if it is central, and one bank if it is a single intake gasket.