Vehicle Inspection, Diagnosis and Maintenance
What this domain tests
This domain tests planned maintenance, systematic diagnosis, road testing, and inspection reporting. Expect questions on service schedules, OBD-II codes and freeze-frame data, compression and leak-down testing, tyre wear patterns, noise and vibration diagnosis, and proper documentation.
Core rules
- A job card records the complaint, mileage, work authorised and parts used; the invoice must match it.
- Read fault codes and freeze frame before clearing anything. Codes P0301 and P0304 indicate misfires on cylinders 1 and 4; P0171 indicates a lean condition on bank 1.
- Compression test: all cylinders should be within about 10% of each other and near specification.
- Tyre wear is diagnostic: centre wear means overinflation, both edges worn means underinflation, one edge worn points to camber or toe, and cupping suggests worn dampers or imbalance.
- Always road test after brake, steering or suspension work, and confirm the repair actually cured the customer complaint.
Worked example
Compression is 11 bar on cylinders 1, 2 and 3 but 7.5 bar on cylinder 4. Add a little oil and repeat: the reading rises to 10.5 bar. Oil seals the rings temporarily, so the fault is worn rings or a scored bore. If the wet reading had stayed at 7.5 bar, the leakage would be at the valves or head gasket. The reasoning is that oil cannot seal a burnt valve or a breached gasket, only the piston ring path.
Traps that cost marks
- Replacing parts purely because a code names them.
- Clearing codes and destroying freeze-frame evidence.
- Overfilling engine oil, which aerates and can blow seals.
- Returning a vehicle without a post-repair road test.