When an air conditioner stops working, many times it's because there's a problem with the machine itself. Occasionally, though, the problem isn't the unit but the thermostat controlling it. Here are three reasons why a thermostat may stop working properly and how to fix the issue.
The Thermostat Is Dirty
Surprisingly, one of the most common reasons why a thermostat will stop working properly—at least temporarily—is because it's dirty. Over time, dust and debris accumulate inside the thermostat and gum up its various moving parts. This can make it difficult for the unit to perform the necessary connections to start/stop the air conditioner and evaluate the ambient temperature.
Therefore, the first thing you should do when determining why your air condition is not working is to clean the thermostat. Turn off the power at the circuit breaker to avoid accidentally electrocuting yourself, and then take the cover off the device. Using a small brush or compressed air, remove dust and dirt from the metal coils, contact plates, and wherever else you find debris. Restore power and then try to turn the air conditioner back on using the thermostat control.
If that resolves the issue, be sure to add regular cleaning of the thermostat to your home maintenance list.
Check the Environment
Another thing that could be causing the thermostat to stop working properly is that it's situated next to a source of excess heat or coldness. The thermostat has a sensor it uses to determine what the ambient temperature is in the home. That's how it knows when to turn the air conditioner on or off. If there is something near the thermostat that causes the air to be hotter or cooler than it is in the rest of the house, that could cause the thermostat to remain on for longer than necessary (resulting in an excessively cool house) or turn off prematurely (causing the home to stay warmer than normal).
Look around the area where the thermostat is situated to see if there's something that may be affecting the temperature around it. If you recently put a fan near the device, for instance, that could be causing the thermostat to register the air as colder than it is, leading it to turn off the air conditioner before it reaches the desired temperature.
If the item causing the issue is not removable—i.e., this issue is caused by a vent—you can work around it by adjusting the temperature to account for the discrepancy. Another option is to have the thermostat moved to a different location. However, this may require the assistance of an electrician as well as an air conditioning repair tech.
There's a Problem With the Wiring
A third problem that can cause thermostat problems is if there is a problem with the wiring between the device and the air conditioner. Wiring problems can arise due to the system's age, deterioration due to mold, or because of pests chewing on the wires.
To determine if the wiring is the issue, eliminate the simple things first. Check to make sure the thermostat is on and that the circuit breaker hasn't been tripped. Sometimes weak batteries won't provide enough power for the thermostat to work; therefore, change them out to eliminate this as the issue.
If the thermostat still won't turn on or if it keeps blowing the fuse or circuit breaker, use a multimeter to test the voltage going to the device. It should read at 24 volts AC. A reading that's higher or lower than that may indicate an issue with the wires. If that's the case, then you'll need to have an electrician take a look at your wiring.
If your air conditioner still continues to malfunction, even after fixing the issue with your thermostat, contact an air conditioning repair technician for assistance.