Diagnosis and Troubleshooting
This domain tests a disciplined diagnostic process. The exam rewards evidence-based isolation over part-swapping, and questions often give a symptom plus several readings.
The process
- Verify the complaint and gather history: when it started, what changed, error codes, recent repairs, usage.
- Reproduce the fault under the conditions described; observe sound, smell, frost, vibration, and sequence.
- Consult the service manual, wiring diagram, and error-code table for the exact model.
- Isolate by half-split: divide the circuit or system into sections and test the boundary.
- Test with the right instrument: voltage under load, current, resistance, temperature, pressure, or microamps.
- Confirm the root cause before replacing parts; then verify the repair under a full cycle.
Useful techniques
- Voltage drop under load finds poor connections that a no-load voltage check misses.
- Wiggle and tap tests, freeze spray, and heat guns can expose intermittent faults.
- Compare nameplate values with measured values; a 10% deviation may be normal, a 50% deviation is not.
- Error codes are clues, not conclusions: a sensor code can be caused by a harness, connector, or board.
- Read the sequence of operation; many faults are a failed step in a sequence, not a failed component.
Worked example
A refrigerator is warm in both compartments; the compressor is silent. You confirm power at the outlet and at the control board. The board has a call for cooling but no voltage at the compressor relay. You check the relay coil and find an open coil. You replace the relay, then check the overload and start components, and verify the compressor starts and pulls normal current. You also check the condenser for lint, because a high head pressure can trip the overload and mimic an electrical failure.
Traps
- Replacing a board because an error code points to it.
- Testing a component without isolating it.
- Assuming a no-cool complaint is a sealed-system fault before checking airflow, defrost, and doors.
- Failing to verify the repair through a complete cycle.