Work through the checks in order. If you smell burning, see smoke, hear arcing, or suspect an immediate electrical hazard, turn the HVAC system off if it is safe to do so and contact appropriate emergency help.
The six-step homeowner check
- 1Thermostat: confirm COOL mode, a setpoint at least three degrees below room temperature, fresh batteries, and fan set to AUTO rather than ON.
- 2Filter: hold it up to a light. If light does not pass through, replace it — this alone fixes many weak-airflow and freeze-up complaints.
- 3Breakers: check both the indoor and outdoor circuits and the outdoor disconnect. Reset a tripped breaker once only.
- 4Float switch: locate the condensate safety switch near the air handler. If the pan is full or the switch is tripped, that is why the system is dead.
- 5Outdoor unit: is the fan spinning? Is the compressor humming? Is the coil visibly caked with dirt, grass or salt film?
- 6Registers and returns: open closed vents, move furniture off return grilles, and look for disconnected flex duct in the attic if one room is much warmer.
Troubleshooting by symptom
- Nothing runs at all → breaker, float switch, thermostat power, low-voltage fuse, transformer
- Indoor fan runs, outdoor unit does not → capacitor, contactor, disconnect, compressor overload
- Outdoor unit runs, indoor blower does not → blower motor, ECM module, control board, float switch
- Both run, air is warm → low refrigerant charge, dirty condenser, failing compressor
- Both run, air is cold but weak → filter, dirty evaporator coil, blower wheel, duct restriction
- Ice on the lines → low airflow or low charge; shut cooling off, run the fan
- Water on the floor or ceiling → clogged condensate drain, rusted pan, failed pump
- Short cycling → oversized system, thermostat placement, low charge, dirty coil
- Loud noises → failing fan motor bearing, loose panel, compressor mechanical failure, debris in the fan
Things you should never do
- Open the electrical compartment of the outdoor unit — capacitors hold a lethal charge even with power off
- Reset a tripped breaker more than once
- Add refrigerant from a store-bought can
- Chip or scrape ice off a coil
- Run the system with an obviously burning smell
- Bypass a float safety switch to keep the system running
Information worth having ready when you call
Note the brand, the approximate age, what the system is doing (and what it is not), when it started, whether anything changed recently, and what you already checked. That short summary regularly shortens the diagnosis and the bill.