Fault Finding, Diagnosis and Repair
Core concepts
Use a structured method: gather the complaint, confirm the symptom, gather data, form hypotheses, test the cheapest and most likely first, then verify the repair under load. Understand common symptom patterns: high superheat with low suction suggests undercharge, restriction or starving evaporator; low superheat with high suction suggests overcharge or excess load; high discharge temperature and pressure suggests condenser issues or non-condensables; frosting at a restriction point suggests a blocked drier or kinked line. Electrical patterns include intermittent overload trips, capacitor degradation, loose terminals causing voltage drop and heat, and sensor drift. Air-side faults such as dirty coils, blocked filters, incorrect fan speed and duct leakage mimic refrigerant faults. Traps include replacing parts on suspicion, ignoring the air side, and failing to check the repair against design conditions.
Traps
Beware questions that describe a single reading and expect a definitive diagnosis; the best answer usually requests more measurement.
Worked example
A cool room runs continuously and will not pull down. Suction pressure is low, superheat high, subcooling low. That combination fits undercharge or a restriction. Check for a temperature drop across the drier; a significant drop confirms restriction. Replace the drier, leak check, evacuate and recharge.
Rule of thumb
One symptom, many causes: diagnose with paired readings (pressure and temperature) rather than a single gauge.