The major function of the Air Charge Temperature sensor, also called Intake Air Temperature Sensor (IAT) monitors the temperature of the air gaining access to the engine. This information is required by the engine computer to evaluate air density, for the sole purpose of balancing air/fuel mixture. Since colder air is denser than hot air, cold air requires more fuel to maintain equal air/fuel ratio. The PCM changes the air/fuel proportion by changing the length of time of the injector pulses.
This sensor is called Air Charge Temperature sensor on pre-OBD II vehicles (1995 & older). It can also be called:
- Vane Air Temperature (VAT) sensor
- Manifold Charging Temperature (MCT) sensor
- Manifold Air Temperature (MAT) sensor
- Charge Temperature Sensor (CTS)
Where is the Air Charge Temperature Sensor Located in a Car?
The Intake Air Temperature Charge (IAT) sensor in a car is located on the bottom of the intake manifold which is directly behind the throttle valve housing.
How Does The Air Temperature Sensor Works?
Normally, the Intake Air Temperature sensor is mounted in the intake manifold in such a way that the tip will be exposed to air gaining access to the engine. For engines that use mass airflow (MAF) sensors to check the air volume entering the engine, the MAP sensor will also have an air temperature sensor built into it. The air temperature sensor is a thermistor, meaning its electrical resistance changes in response to temperature changes.
The air temperature sensor functions in same as a coolant sensor does. The PCM applies a reference voltage to the sensor, then observes the voltage signal it gets back to compute air temperature. The signal of the return voltage will change in the amount as a response to changes in air temperature. Most air temperature sensors are negative temperature coefficient (NTC) thermistors with high electrical resistance when they are cold, however, the resistance drops as they heat up. Some function in the opposite manner. There is also a positive temperature coefficient (PTC) thermistors that have quite a low resistance. The fluctuating resistance of the sensor causes a change in the return voltage back to the PCM.
The Automatic Climate Control systems also use Air temperature sensors. One or more air temperature sensors are used to monitor the temperature of the air inside the passenger compartment and the outside air temperature. The climate control system usually has its own separate outside air temperature sensor located outside the engine compartment so it is not affected by engine heat. The outside air temperature sensor is normally mounted behind the grille or at the base of the windshield. Majority of these sensors work just the same as the engine air temperature sensor. However, some use an infrared sensor to check the body temperature of the vehicle’s occupants.
What Causes an Air Temperature Sensor to Fail?
- An air temperature sensor can sometimes be damaged by miscarrying or backfiring in the intake manifold.
- The tip of the sensor can also be coated by carbon and oil contamination inside the intake manifold. When this happens, it makes it less responsive to sudden changes in air temperature.
- The air temperature sensor may also deteriorate with age or heat, leading to a slower or no response.
- Poor electrical connections at the sensor can also lead to sensor problems. A loose or rusty wiring connector can affect the output of the sensor. A damaged wiring in the circuit between the sensor and PCM can also lead to malfunction.
What are the Symptoms of a Failed Air Temperature Sensor?
If the intake air temperature sensor fails to read accurately, the PCM may think the air is warmer or colder than it actually is, causing it to misjudge the air/fuel mixture. This can lead to a lean or rich fuel mixture that causes drivability symptoms. These signs may include:
- stumble on cold acceleration
- poor idle quality when cold
- surging when the engine is warm
If the sensor isn’t reading accurately when the engine computer uses the air temperature sensor input to turn on a cold start injector, it may thwart the cold start injector from being operational, hence leading a hard cold start condition.
A defective air temperature sensor may affect the operation of the EGR valve if the PCM uses air temperature to control when the EGR valve opens. A faulty air temperature sensor reading warmer than normal will naturally lead to a lean fuel condition, thereby increasing the risk of detonation and lean misfire.
A faulty air temperature sensor with colder than normal reading will typically cause a rich fuel condition which leads to fuel wastage and hence increases wastage and emissions.
Sometimes a fuel mixture balance problem due to a faulty air temperature sensor may be caused by an engine vacuum leakage or a restricted catalytic converter.
How Can The Air Temperature Sensor Be Diagnosed?
There’s a probability of a faulty air temperature sensor setting or not setting a code and turning on the Check Engine light. A code will usually be set if the sensor circuit is open or shorted. However, if it is only reading high or low, or is slow due to old age or contamination, it will usually not set a code.
You can use a scan tool to compare the air temperature reading to the coolant temperature reading when the engine is warm. This is the quickest way to check the air temperature sensor. A good air temperature sensor will typically read a few degrees cooler than the coolant sensor.
Apart from the method above, you can also check the sensor’s resistance using an ohmmeter. This can be done by removing the sensor and connecting the two leads on the ohmmeter to the two pins in or on the sensor’s wiring connector plug to measure the resistance of the sensor. Measure the sensor’s resistance when cold, then blow hot air at the tip of the sensor with a blow drier and measure the resistance again. Check for a change in the reading of the resistance as the sensor warms up.
If there’s no change in the resistance reading as it warms up, then it indicates that the sensor is bad and needs to be replaced. If the sensor reading gradually decreases, it means the sensor is a negative thermistor. If it gradually increases, then it is a positive thermistor. You may have a bad sensor if the reading suddenly goes open or shorts out.
Warning: Never use a propane torch to measure the sensor’s resistance! You should only use a blow-drier.
To be more accurate, look up the resistance specifications for the air temperature sensor, then measure the sensor’s resistance at low, mid-range and high temperatures to check if it corresponds to the specifications. A sensor that reads within the specified range when cold, may eventually go out of range when the temperature is high. There would be no need to replace such a sensor since it would not be accurate.
How to Replace, Repair, and Adjust Air Temperature Sensor
It is impossible to adjust the air temperature sensor since it is a solid-state device. However, a dirty sensor may be cleaned so that it functions normally as long as it is still in a good functional state. You can get rid of contaminants from the tip of the sensor by:
- removing the sensor from the intake manifold
- using electronics cleaner to spray the sensor tip
For sensors mounted inside a MAF sensor, the wire sensing element can also be sprayed with aerosol electronics cleaner. Avoid using any other type of cleaner because it may destroy the plastic housing or leave behind a chemical residue that may eventually lead to faults later on.
You should replace a sensor that has failed, or that is not reading within specifications. Luckily, most air temperature is not very expensive and the workmanship charged to change the air temperature sensor is usually not high unless the sensor is buried under a lot of other stuff that would warrant it to be removed.
Exercise caution when replacing the air temperature sensor. Do not tighten it too much because it may damage the threads in a plastic intake manifold or destroy the sensor housing.