Mechanic Glossary

A/C Evaporator Temp Sensor

A sensor that monitors the temperature of the evaporator core to prevent it from freezing over with ice.

The A/C evaporator temperature sensor (or freeze switch) is a thermistor mounted directly on or inside the evaporator core. It monitors the core temperature. If the temperature drops close to freezing (32F), the sensor sends a signal to the engine computer to cycle off the compressor clutch. This prevents moisture on the fins from freezing into a solid block of ice.

If this sensor fails, the compressor might run constantly, causing the evaporator to freeze over. When this happens, ice will block all airflow through the cabin vents. Alternatively, a failed sensor can prevent the compressor from turning on at all.

Frequently Asked Questions

The cabin vents will stop blowing air after 15-20 minutes of driving because the evaporator is frozen, or the A/C compressor will refuse to turn on.
Moisture in the air condenses on the cold evaporator fins. If the core gets too cold, this water freezes into a solid wall of ice, blocking the passage of air from the blower motor.
Measure the sensor's resistance with a digital multimeter while exposing the sensor tip to ice water. The resistance should change smoothly according to the manufacturer's temp chart.
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