Refrigeration System Diagnostics and Troubleshooting
Domain 3: Diagnostics and Troubleshooting
The exam tests a repeatable diagnostic sequence for no-cooling, warm product, icing, short cycling, and abnormal pressures. Strong answers separate airflow, electrical, refrigerant charge, and control faults before condemning parts.
Core rules
- Start with the complaint and product temperature, then verify airflow: evaporator fans, return-air path, dirty coil, and defrost termination.
- Measure suction and liquid pressures, convert to saturation temperatures, and calculate superheat and subcooling at steady conditions.
- Use air temperature difference across the evaporator and condenser as supporting evidence; compare it with design and confirm with superheat rather than diagnosing from air temperature alone.
- A restriction shows a temperature drop across the filter-drier or TXV inlet screen; frost or sweat at the restriction is a clue.
- Non-condensables raise discharge pressure and subcooling; a dirty condenser raises condensing temperature and amp draw.
- Defrost failures show as ice on the evaporator, short run times, or a defrost cycle that times out.
Worked example
A medium-temperature display case is warm. Suction pressure is low, superheat is 35°F, subcooling is 2°F, and the sight glass shows bubbles. The filter-drier inlet is 95°F and the outlet is 78°F, a 17°F drop. Reasoning: low superheat would suggest flooding, but high superheat plus low subcooling means the evaporator is starved. The 17°F drop across the drier confirms a restriction, not just low charge. Recover refrigerant, replace the drier, evacuate, and charge by weight; then verify superheat returns to the TXV setting.
Traps that cost marks
- Adding refrigerant to clear bubbles before leak-checking.
- Diagnosing a TXV when the real fault is a restricted drier or low airflow.
- Using a single pressure reading without converting to saturation temperature.
- Ignoring the defrost timer and termination stats on iced coils.