AC Repair & Diagnostics

Superheat and subcooling are the charge diagnosis. Superheat measures how much the refrigerant vapor has warmed past its boiling point leaving the evaporator; subcooling measures how much the liquid has cooled below condensing temperature leaving the condenser. Together, read against outdoor and indoor conditions, they tell us whether the system is undercharged, overcharged, restricted at the metering device, or short on airflow — four different faults that all present as weak cooling.

Temperature split — return air temperature minus supply air temperature — should typically land in the high teens to low twenties on a properly operating residential system. Too small a split points at low charge, a fouled coil, or a compressor losing capacity. Too large a split usually means airflow is restricted.

Static pressure is the duct system's report card: measured across the equipment and compared to the design figure, often around 0.5 inches of water column. Elevated static means the blower is fighting the ductwork, which explains frozen coils, weak delivery, and blower failures all at once.

Capacitor microfarads measured against the rating printed on the can. A run capacitor drifting more than about 10% below rating is on its way out, and it is the single most common failure in this climate because high ambient temperatures accelerate electrolytic degradation. Measuring it during a maintenance visit is how a July emergency becomes an April repair.

Amp draw on the compressor, condenser fan, and blower compared to nameplate values. High draw means a motor working against a mechanical problem, a bearing failure, or a compressor in trouble. Low or zero draw with voltage present points at a winding failure or a control problem.

Then the control path: voltage at the contactor coil, contact condition, transformer output, low-voltage fuse, thermostat call, float switch continuity, and safety controls in series. A system that does nothing at all is nearly always a break somewhere in that chain, and tracing it in order takes minutes when done systematically.

Blower runs, outdoor unit silent, capacitor reads well below rating: failed run capacitor. Outdoor unit silent, capacitor good, no voltage at the contactor coil: control path — thermostat, transformer, fuse, or a tripped float switch. Outdoor unit humming with high amp draw and no start: compressor or start components.

Low superheat with normal subcooling and elevated static pressure: airflow restriction — a loaded filter, a dirty evaporator coil, or a duct problem — and the fix is airflow, not refrigerant. High superheat with low subcooling: undercharged, meaning a leak that needs finding. Normal charge readings with a small temperature split and a plugged condenser: a coil cleaning problem, common every Valley summer.

System cooling for twenty minutes then stopping, with water in the pan and a tripped float: plugged condensate drain, the most common monsoon-season call we take. Ice on the line set: shut cooling off, run the fan to thaw, then diagnose airflow and charge — because running it iced sends liquid refrigerant to the compressor and turns a cheap fault into an expensive one.