If you cannot get the air conditioning system to activate at all, then the most common causes lie with a blown circuit breaker or fuse, improperly set or faulty thermostat or an internal switch being off.
- Ensure the thermostat itself is in the “Cool” position and not set to “Off” or “Heat.”
- Ensure the thermostat is calling for cooling by making sure the thermostat is set below the current room temperature.
- Check that the 240 volt circuit breaker (double breaker) controlling the air conditioning compressor / condensing unit and the 120 volt circuit breaker controlling the furnace blower or separate air handler, are in the “on” positions. If a circuit breaker has popped or a fuse is blown, then reset the circuit breaker or replace the fuse. If you reset the breaker or replace the fuse and they fail again, stop and call an air conditioning service technician as you may have a more serious problem.
- Check that all switches in and around the air conditioner are set to the “on” position including the external safety switch which is usually on an outside wall next to the condensing unit.
- Check the condensate overflow tray (if your unit has one) for excessive water. Sometimes this tray is installed in remote air handlers using condensate collection instead of a condensate drain. When using a tray, there may be a sensor switch that turns the unit off when water collects in the tray.
- Check to make sure the blower door on the air handler is securely closed.