Checking basic thermostat settings, power supply components, and air filter conditions are essential to troubleshoot why an AC unit isn't turning on. More advanced issues requiring professional attention include faulty wiring, a malfunctioning outdoor unit, a bad capacitor, or incorrect thermostat calibration.
One of the most common reasons why a thermostat may fail to turn on the air conditioning system is dead batteries. If the batteries in your thermostat have died, it may not have enough power to communicate with the air conditioning system and turn it on.
First check fuse. Then check all batteries to thermostat. Weak batteries will not engage thermostat. Then check voltage to see if voltage is going out to outdoor unit. If not, take off one of the thermostats and jump the cooling on. This will force the ac to turn on.
A few things may go wrong that cause a thermostat to turn unresponsive: Loss of direct connection to the air handler or compressor. There are separate wires from the thermostat to the parts of the air conditioner, and if one of them frays or breaks, the AC may not turn on (or may not turn off).
Air Conditioner Won't Turn On
Be sure the thermostat is set to “cool” mode and the temperature is set low enough to call for the A/C to come on. Look for a tripped circuit breaker or blown fuse. If you're satisfied that the system is getting power, a faulty thermostat may be preventing the system from turning on.
HVAC System Suddenly Quits Working
Try replacing the batteries, then check for a blown fuse or tripped breaker in the electrical panel. If the thermostat is still unresponsive, make sure the breaker is shut off and remove the cover.
The main reasons your AC unit outside is not turning on could be something that has tripped the circuit breaker, something is blocking the fan's airflow, or the condensing unit is damaged. Other potential issues could be the thermostat isn't set correctly or the compressor could be malfunctioning.
Switch the thermostat to the off position. Locate the breaker that powers your HVAC system and turn it off. Wait 30 seconds and turn the breaker back on. Turn your thermostat back on and confirm it has reset properly.
However, if the thermostat is not working properly, it can lead to several issues, including inadequate cooling.
To test them, switch your thermostat “off,” and then switch back to “on.” Set it to the heating or cooling mode, and turn it up or down, depending on the season. Some thermostats will make a clicking sound when they turn on — you might even hear your HVAC system click on as well.
It Won't Turn On
Check your breaker box for a blown breaker or tripped fuse. You may be able to fix this AC issue with the flip of a switch. If that doesn't do the trick, check the wiring in your thermostat. A loose or frayed wire could also prevent your unit from turning on.
Try checking the circuit breaker box or replacing the batteries. Bad wiring: The thermostat may be unresponsive if the wiring has gone bad. Remove the cover and check inside for loose, detached, or corroded wires. If you spot any problems, contact an HVAC technician for assistance.
If your AC is running but not cooling, it could be due to a dirty air filter, incorrect thermostat settings, a malfunctioning thermostat, a refrigerant leak, dirty condenser coils, blocked vents or ducts, a faulty compressor, frozen evaporator coils, or electrical issues.
Check the power source and make sure it is connected and turned on. Verify that the circuit breaker for the AC unit is not tripped. Make sure the thermostat is set to 'COOL' mode. Check the fan setting on the thermostat and ensure it is set to 'AUTO' rather than 'ON'.
Make sure the outside unit is turned on (there's probably a switch near the unit). If the outside unit has a separate circuit breaker, check your circuit box and make sure it hasn't been tripped. A more serious issue if the outside air conditioning unit's not working is a broken compressor, capacitor or fan.
Check Your Circuit Breaker
Locate your main circuit breaker and switch it back to the "On" position. Wait 30 seconds and turn on your air conditioner again. If it still doesn't work, you'll need to call a professional for help.
Press the reset button. Most models require you to hold the button down for 3-5 seconds before releasing. Restore power to your AC. Restore power to the unit at the circuit breaker, and make sure the unit is set to “on” and your thermostat is set to “cool.”
Resetting your thermostat is often all that's necessary to get it working again. A tripped circuit breaker is another common reason why your AC won't turn on. Using too many appliances at once can trip the circuit breaker. This is also common when there are power surges after a big storm.