Troubleshooting and Service
Troubleshooting and Service
This domain tests a disciplined diagnostic sequence using pressures, temperatures, electrical readings, and airflow. Strong answers separate symptoms from root causes.
Core rules
- Confirm the complaint, then verify power, thermostat demand, safeties, and airflow before connecting gauges. A dirty filter can mimic a refrigerant fault.
- Compare measured superheat and subcooling with manufacturer targets at the same indoor and outdoor conditions.
- Low suction pressure with high superheat and low subcooling suggests undercharge or a restriction. A restriction often shows a temperature drop across the filter drier or TXV.
- High head pressure can come from a dirty condenser, failed fan, overcharge, or noncondensables. High suction pressure can come from high load, low airflow, or an overfeeding metering device.
- Electrical checks: verify compressor winding resistance, capacitor microfarads, contactor pull-in, and voltage drop under load. Use lockout/tagout.
- Do not bypass high-pressure, low-pressure, or float safeties to "test" a system.
Worked example
A call reports poor cooling. Outdoor unit runs; suction pressure is low, superheat is 25°F, subcooling is 4°F, and the liquid line is warm. Reasoning: low subcooling and high superheat indicate the condenser is not backing up liquid and the evaporator is starved. That pattern fits undercharge more than a liquid-line restriction, which would usually increase subcooling. Leak-check the system, repair, evacuate, and weigh in charge. Do not simply add refrigerant, because the leak will remain.
Traps
Adding charge to fix low airflow; reading superheat before the system stabilizes; using the wrong refrigerant chart; and condemning a compressor without checking the run capacitor and contactor. Also, a frozen suction line can indicate airflow, not charge.