Diagnosis, Repair, and Maintenance
This domain tests how you think: how you verify a complaint, isolate a fault, choose a repair, and prove it works. Questions often present a symptom and ask for the next diagnostic step rather than the answer.
A repeatable diagnostic process
- Verify the complaint and gather history.
- Inspect visually before testing.
- Test to isolate the system, then the component.
- Identify root cause, not just the failed part.
- Repair or replace per the OEM procedure.
- Verify the repair and document it.
Skipping verification is the most common cause of comebacks. Asking why a part failed - the 5 Whys - prevents repeat failures such as a new chamber failing because the relay valve is passing oil.
Preventive maintenance and inspection
PM schedules combine mileage and time intervals. CVSA roadside inspections apply out-of-service criteria for brakes, tires, wheels, coupling, and lighting, so maintenance must meet those thresholds, not merely "work." Record torque values, measurements, and parts traceability so the next technician can follow the story.
Tools for diagnosis
Scan tools for ABS codes, pressure gauges for air systems, multimeters for electrical faults, flow meters for air delivery, and temperature checks for bearings and brakes. Use the tool that measures the condition rather than the one that is closest.
Worked example
An ABS lamp illuminates intermittently on rough roads. Reasoning: an intermittent fault suggests a connection or gap issue rather than a failed ECU. Retrieve blink codes or scan data, then inspect the wheel speed sensor gap, tone ring condition, and harness routing for chafing. A sensor that reads correctly on a smooth road but drops out over bumps points to a loose mounting or damaged harness.
Traps that cost marks
Replacing parts without isolating the fault, failing to verify the repair, skipping documentation, and ignoring the root cause. Also watch for answers that adjust a component to compensate for wear instead of replacing it.