The air cleaner temperature sensor (ACT) plays an important role in many modern vehicle’s engine management systems. The ACT sensor detects the temperature of the air coming into the engine and sends a signal to the computer so that it can make fuel and timing adjustments based on the operating conditions detected by the sensor. When the sensor begins to have problems, it can send an incorrect signal to the computer which can negatively affect engine performance, so it should be inspected and replaced if needed. When the air cleaner temperature sensor fails, the vehicle will usually display a few symptoms that can alert the driver of a potential problem.

1. Poor idle quality

Poor idle quality is one of the first symptoms of a problem with the air cleaner temperature sensor. The ACT sensor provides a signal that is very important to the engine computer calculations for proper idle conditions, especially during cold starts and in cold weather conditions, where the incoming air density is increased. When the sensor is having problems, it can send an incorrect signal to the computer which may cause the engine to have a low, rough, or surging idle.

2. Engine performance problems in cold conditions

The ACT sensor detects the temperature of the air coming into the engine, so that the computer can make the correct calculations for best engine performance. This signal becomes even more important in cold and wet weather conditions as cold air is significantly more dense than warm air. If the ACT sensor is having problems, the vehicle may have trouble idling, or may stumble and misfire when accelerating after cold starts, or during cold or wet weather.

3. Check Engine Light comes on

Aside from drivability symptoms the most obvious sign of a problem with the ACT sensor is a Check Engine Light. If the computer detects a problem with the signal of the sensor, the light will activate. This is usually the last symptom to occur, as it only activates once a problem has been detected. A quick scan for the trouble codes will quickly reveal what the problem may be.

As the ACT sensor provides an important signal to the computer, any problems with it can quickly translate to engine performance problems. If you suspect that you may be having a problem with you ACT sensor, or your Check Engine Light has come on, have a professional technician, such as one from YourMechanic, diagnose the vehicle and replace the ACT sensor if necessary.

This article originally appeared on as Symptoms of a Bad or Failing Air Cleaner Temperature Sensor and was authored by Eduardo Ruelas.

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