Diagnostics, Maintenance, Service and Repair
What this domain tests
The final domain tests systematic fault finding, scheduled maintenance, service checks, repair decisions and road testing. Questions expect a logical sequence: verify the complaint, inspect, test, repair, confirm. You must know when to use compression, leak-down, vacuum, spark and scan-tool tests.
Core rules
- Confirm the customer complaint and operating conditions before touching the machine. Record DTCs before clearing them.
- Follow a sequence: visual inspection, basic checks, functional tests, measurements, then parts replacement. Never replace a component without a test that supports the decision.
- Compression test: hold the throttle open, crank a set number of strokes and compare with specification. Low compression can come from rings, valves or head gasket; a leak-down test locates the leakage path.
- Spark test confirms ignition presence, not quality; check plug condition, gap, cap resistance and coil supply.
- Fuel diagnosis checks pump delivery, injector pulse, pressure and contamination.
- Maintenance includes valve clearance, oil and filter, air filter, brake fluid, coolant, chain, tyre pressure, fasteners and road test.
Worked example
A motorcycle cranks but does not start. Battery reads 12.6 V at rest and 10.5 V while cranking, which is acceptable. Spark is present and fuel reaches the rail. Compression measures 90 psi against a 150 psi specification. A leak-down test shows air escaping from the intake, indicating an intake-valve leak. The correct repair is valve service, not ignition-coil replacement.
Traps that cost marks
Clearing codes before recording freeze-frame data; condemning a battery on voltage alone; adjusting valve clearance on a hot engine when cold is specified; skipping the road test after brake work; using a compression gauge with the throttle closed.
Key points
- Verify the complaint before diagnosis.
- Record DTCs before clearing them.
- Compression finds low pressure; leak-down locates it.
- Replace parts only after a supporting test.
- Road test confirms the repair and safety.