The evaporator temperature sensor or switch is a component commonly found on many road going vehicles. It is a part of the vehicle’s AC system, and as its name implies, detects and monitors the temperature of the AC system’s evaporator. The switch is usually installed directly on the AC evaporator or in close proximity, and monitors the temperature of the evaporator so that it can be properly regulated to stay within reasonable operating temperatures. When the sensor detects that the temperature is approaching high temperatures, the AC system will increase flow through the system to increase cooling and will reduce flow when it detects that the evaporator is cold to conserve power and prevent it from freezing over. When the switch fails or has any issues, it can hinder the performance of the AC system and even cause it to malfunction. Usually a bad or failing evaporator temperature switch will produce a few symptoms that can alert the driver of a potential issue that should be serviced.

1. No cold air from the AC system

One of the first symptoms of a potential problem with the evaporator temperature switch is no cold air from the AC system. If the evaporator temperature switch fails or has any issues that causes it to send an incorrect signal to the AC system, it can affect the system’s ability to produce cold air.

2. AC temperature fluctuates

Another symptom of a problem with the evaporator temperature sensor is fluctuating temperatures from the AC system. If the evaporator temperature sensor sends an incorrect or erratic signal to the computer, it can cause the AC temperature to behave erratically. The temperature may suddenly alternate between cold and warm, which will result in additional strain on the compressor, as well as consistently less cool cabin temperatures.

3. AC compressor does not activate

An AC compressor that does not activate is another symptom of a possible problem with the evaporator temperature sensor. The AC switch plays a direct role in activating and regulating the operation of the compressor, and when it fails, can sometimes even disable the compressor completely.

The evaporator temperature sensor plays a simple, but important role for the proper operation of the AC system. When the switch fails or has an issue, it can cause problems with the functionality of the entire AC system. If you suspect that your evaporator temperature switch may be having an issue, have the vehicle diagnosed by a professional technician, such as one from YourMechanic, to determine if your vehicle needs an evaporator temperature sensor replacement.

This article originally appeared on as Symptoms of a Bad or Failing Evaporator Temperature Sensor (Switch) and was authored by Ed Ruelas.

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