Diagnosis, Service, and Repair
What this domain tests
Diagnosis, service, and repair questions ask you to move from symptom to root cause using measurements and a sequence, then repair and verify. The exam favors systematic troubleshooting over parts swapping.
Core rules and concepts
- Verify the complaint and operating conditions. Record suction and liquid pressures, line temperatures, ambient, and box temperature.
- Calculate superheat and subcooling and compare them with targets. Check airflow, coil condition, defrost operation, and controls.
- Low suction pressure with high superheat suggests undercharge, a restriction, or low load. High suction pressure with low superheat suggests overcharge, flooding, or high load.
- High head pressure with high subcooling suggests overcharge, noncondensables, or a dirty condenser. High head pressure with normal subcooling points to airflow or ambient issues.
- Leak detection may use electronic, UV, or soap methods. After repair, pressure-test, evacuate, replace the filter-drier, and weigh the charge.
- Compressor replacement requires matching voltage, phase, RLA, oil type, piping, and electrical components, followed by evacuation and verification.
Worked example
A medium-temperature cooler runs continuously and holds 45°F. Suction pressure corresponds to 15°F, and the suction line is 45°F, so superheat is 30°F. Liquid subcooling is only 2°F, and the sight glass shows bubbles. There is no frost at the TXV. The likely causes are low charge or a liquid-line restriction. Check the filter-drier for a temperature drop and inspect the TXV screen. If no restriction is found, leak-search, repair, evacuate, and weigh the charge. Do not simply add refrigerant to raise suction pressure.
Traps that cost marks
Adding refrigerant without finding the cause; replacing a compressor for a control problem; failing to replace the filter-drier after a burnout; skipping the evacuation decay test; and not checking defrost termination before condemning the refrigeration circuit.
Key points
- Start with pressures, line temperatures, ambient, and box temperature.
- Superheat and subcooling point to charge, restriction, or airflow problems.
- High superheat with low suction means investigate charge and restrictions.
- After repair: leak-test, evacuate, replace drier, and weigh charge.
- Verify defrost and controls before replacing refrigeration components.