HVAC systems are the most expensive appliance in most homes, and they fail at the worst times. The list below covers the diagnoses you can do safely as a homeowner — anything involving refrigerant or sealed combustion needs a licensed technician. Always shut off power at the disconnect (outdoor unit) and the breaker (indoor air handler/furnace) before service.
In order of likelihood: dirty air filter (replace every one to three months — this single thing prevents half of HVAC failures), thermostat set wrong or batteries dead, outdoor disconnect tripped or breaker tripped, dirty outdoor condenser coils (rinse with a garden hose from the inside out, top down), or low refrigerant charge (technician). Frozen evaporator coil at the indoor unit is also common — caused by airflow restriction or low refrigerant — you'll see ice on the copper line at the air handler.
Failed blower motor, failed blower capacitor, frozen evaporator coil, or disconnected ductwork. Open the air handler door — if the blower wheel isn't spinning while cooling is called, the motor or capacitor has failed.
Blown contactor (the relay that closes when cooling is called — they pit and stick), failed start capacitor (the cylindrical can on top of the unit — bulged or leaking means failed), thermostat not calling, or blown disconnect fuse. Capacitors are the single most common AC failure and a $20 part.
Gas valve closed, dirty flame sensor (the most common failure — pull and clean with very fine sandpaper or a Scotch-Brite pad), failed igniter, blocked condensate drain (high-efficiency furnaces shut off when the drain backs up), or pressure switch problem. Watch through the burner sight glass: igniter glows orange, gas valve opens, flame lights, then either runs or shuts off after a few seconds. Shut-off after lighting is the flame sensor.
Blower motor not running, broken belt (older units), or wrong fan setting on thermostat. If the burner runs but the blower doesn't start, the limit switch will eventually open and shut off the burner — repeated cycling is hard on the heat exchanger.
Failed compressor (drawing locked-rotor amps), shorted contactor, or seriously failed capacitor. Don't keep resetting — you'll damage other components. This usually needs a technician.
Bad blower motor bearings, failing compressor bearings (outdoor unit), or loose duct or panel. Hum from the outdoor unit with no fan motion is a failed fan capacitor.
Clogged condensate drain. Pour a cup of bleach into the drain pan port (where the PVC line attaches), then suction the line with a wet/dry vac at the outdoor termination. High-efficiency furnaces have multiple drain points that all clog.
Most modern thermostats need a C-wire for power; running the system on AA batteries alone often causes erratic behavior. Replace batteries first. Persistent communication errors on smart thermostats usually trace to bad terminations at the air handler.
Look at the small viewing window on the furnace cabinet — there's an LED that flashes a code. The legend is on the inside of the panel or in the manual on this site. Common codes: pressure switch, flame sense, ignition lockout, limit switch open.
Anything involving refrigerant requires an EPA-certified technician — including diagnosing a low charge. Heat exchanger inspections on gas furnaces require combustion analysis equipment. Carbon monoxide concerns are an immediate technician call regardless of whether the system is running.
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