The air conditioner operates in cycles that means that it will turn itself on or off based on the temperature preferences. However, a malfunctioning AC might run endlessly. The endless running can accelerate wear and tear while wasting energy. Below are some of the reasons an AC might fail to turn off.

1. ECB Malfunctions

Modern ACs are electronically controlled. The electronic controls are located in an integrated circuit known as an electronic control board (ECB)—a miniature computer. The ECM controls signals between the thermostat and the other parts of the AC, including the fan motor.

For example, once the AC has cooled your house to the required temperature, the thermostat sends signals to the ECB that instructs the motor to turn off. An electronic malfunction of the ECB will prevent this function, which might make the AC continue running even if the target temperature has been reached.

2. Thermostat Malfunctions

The thermostat is central to the AC's cooling function; it detects the room temperature and sends the signals to the ECB for onward AC control. The thermostat can malfunction due to dust exposure, wise disconnections, corrosion, and other forms of wear and tear. If that happens, the thermostat might fail to pick up the room temperature or send the correct signals to the ECB. Again, the AC will keep running even if the room temperature is cool enough.

3. Stuck Relay

The AC relay is an electronic switch known as a relay. The relay has points of contact that touch to switch on the AC and detach to turn on the AC depending on the set cooling needs or temperature. The usual relay malfunction is for the contact points to wear off and prevent the AC from switching on. However, it is also possible for the relay to get stuck on the 'On' position. A struck relay is like a switch that cannot be turned off, which means the AC will keep running irrespective of the house temperature.  

4. No Cooling

A well-functioning AC only runs if the house temperature is higher than the temperature setting on the thermostat. The AC automatically switches off once the house temperature and thermostat temperature match. This means the AC might not switch off if it is struggling to cool the house since the two temperatures won't match. Typical causes of impaired cooling include clogged filters, frozen coils, and blocked registers, among other things.

As you can see, there are many reasons an AC might have long run cycles. Consult an AC or heating contractor for a diagnosis and solution if you are facing such problems.