Most common causes
- Thermostat problem. Dead batteries, the wrong mode, a schedule overriding your setting, or a loose wire can stop the call for cooling from ever reaching the system.
- Tripped breaker. The AC usually has its own breaker in the main panel, and the furnace or air handler has another. Either one tripping can stop the system.
- Outdoor disconnect off. The disconnect box next to the condenser can be switched off by accident, for example during yard work.
- Float switch tripped. If the condensate drain is clogged, a safety float switch cuts power to the AC so it does not flood the house.
- Furnace door switch open. Many indoor units have a safety switch that keeps the system off if the blower door is not fully closed.
- Failed electrical part. A blown fuse on the control board, a bad transformer, capacitor, or contactor can keep the system dead even with power present.
What you can safely check
- Set the thermostat to Cool, lower the setpoint below room temperature, and replace the batteries.
- Check the main electrical panel for tripped AC or furnace breakers. Reset once only.
- Check that the outdoor disconnect is On and that the switch near the furnace or air handler (it looks like a light switch) is On.
- Look at the condensate drain pan and float switch. If water is backed up, clear it before expecting the system to start.
- Make sure the furnace or air handler panel door is fully closed so the door switch is pressed.
- After any reset, wait 5 to 10 minutes. Many systems have a built-in delay.
Call a technician if
- A breaker trips again after resetting
- The thermostat screen is blank even with new batteries
- The indoor unit runs but the outdoor unit does not start, or the reverse
- You smell burning or see scorch marks
- Everything has power and correct settings and it still will not start
What affects the repair cost
If it is a setting, batteries, or a tripped switch, there may be no repair cost at all. Electrical repairs vary with the part: fuses and capacitors are small parts, while a control board or thermostat replacement costs more. Wiring damage from pests or age depends on how much needs to be replaced.
