Troubleshooting, Maintenance, and Repair
Air side first, then refrigerant side
Use a consistent order: verify the complaint, check thermostat settings and power, confirm airflow through filter, blower, belt, and coil, then measure pressures, temperatures, superheat, subcooling, temperature split, static pressures, and amp draw. Most no-cooling calls are air-side problems, such as a loaded filter, dirty evaporator, collapsed flex duct, or closed damper. Adding refrigerant makes those worse.
Recognize the classic patterns. Low suction with high superheat means undercharge or a restriction. High suction with low superheat means overcharge or an overfeeding metering device. High head with high subcooling means a dirty condenser, non-condensables, or overcharge. Frost or a temperature drop at the filter drier means a restriction there. Compressor valve failure shows an abnormal compression ratio with unremarkable amps; confirm with pump-down or performance data rather than assumption.
Maintenance tasks include coil cleaning, belt tension and alignment, bearing lubrication, condensate trap and pan, economizer linkage and sensors, refrigerant leak checks, electrical torque checks, and recording readings against a baseline so trends are visible.
Traps: condemning a compressor before testing the capacitor and contactor; charging without verifying airflow; ignoring the temperature drop across a drier; skipping the economizer and second-stage test.
Worked example: a 20 F temperature split with 12 F subcooling and normal pressures suggests the equipment is performing; the complaint is probably distribution or insulation. Measure supply and return at the equipment, not at a distant register.