Troubleshooting, Service, and Maintenance
This domain tests symptom-based diagnosis, electrical and refrigerant troubleshooting, and maintenance. Expect questions that link a complaint such as “not heating” or “iced coil” to airflow, charge, reversing valve, defrost, or electrical causes.
Core rules and concepts
- A frozen indoor coil usually points to low airflow, low charge, a restricted metering device, or a blower problem. Correct airflow first.
- Low suction pressure with high superheat suggests undercharge or a restriction. High suction pressure with low superheat suggests overcharge or an inefficient compressor.
- High head pressure can come from a dirty condenser, failed fan, overcharge, or noncondensables.
- A reversing valve that will not shift may have a failed solenoid, a board output problem, or a mechanically stuck valve. Check 24V at the solenoid and temperature difference across the valve.
- Inspect capacitors, contactors, defrost sensors, drain safety switches, and compressor windings. Pitted contactors and weak capacitors cause intermittent failures.
- Maintenance includes filter changes, coil cleaning, drain clearing, electrical torque checks, and leak inspection.
Worked example
A heat pump runs in heating but produces little heat. Suction pressure is low, head pressure is low, and the outdoor coil is not frosted. The compressor is running. Check whether the reversing valve is stuck in the cooling position by verifying solenoid voltage and the temperature difference across the valve. If 24V is present at the solenoid but the valve does not shift, the valve may be mechanically stuck. If no voltage is present, check the defrost board and thermostat before replacing the valve.
Traps that cost marks
Adding refrigerant before checking airflow; replacing a compressor without checking the capacitor and contactor; misdiagnosing a stuck reversing valve as a charge problem; and ignoring a clogged condensate drain safety switch.