The Intake Air Temperature sensor (IAT) monitors the temperature of the air entering the engine. The engine computer (PCM) needs this information to estimate air density so it can balance air air/fuel mixture. Colder air is more dense than hot air, so cold air requires more fuel to maintain the same air/fuel ratio. The PCM changes the air/fuel ratio by changing the length (on time) of the injector pulses.
On pre-OBD II vehicles (1995 & older), this sensor may be called an Air Charge Temperature (ACT) sensor, a Vane Air Temperature (VAT) sensor, a Manifold Charging Temperature (MCT) sensor, a Manifold Air Temperature (MAT) sensor or a Charge Temperature Sensor (CTS).
HOW THE AIR TEMPERATURE SENSOR WORKS
The Intake Air Temperature sensor is usually mounted in the intake manifold so the tip will be exposed to air entering the engine. On engines that use mass airflow (MAF) sensors to monitor the volume of air entering the engine, the MAP sensor will also have an air temperature sensor built into it. Some engines may also have more than one air temperature sensor (two if it has a split intake manifold or separate intake manifolds on a V6 or V8 engine).
The air temperature sensor is a thermistor, which means its electrical resistance changes in response to changes in temperature.
It works the same as a coolant sensor. The PCM applies a reference voltage to the sensor (usually 5 volts), then looks at the voltage signal it receives back to calculate air temperature. The return voltage signal will change in proportion to changes in air temperature. Most air temperature sensors are negative temperature coefficient (NTC) thermistors with high electrical resistance when they are cold, but the resistance drops as they heat up. However, some work in the opposite manner. They are positive temperature coefficient (PTC) thermistors that have low resistance when cold, and increase in resistance as they heat up. The changing resistance of the sensor causes a change in the return voltage back to the PCM.
On older pre-OBD II applications (1995 & older vehicles), the signal from the air temperature sensor may also be used to turn on the cold start injector (if used) if the outside air temperature is cold. On some of these older applications, the air temperature sensor signal may also be used to delay
the opening of the EGR valve until the engine warms up.
Air temperature sensors are also used in Automatic Climate Control systems. One or more air temperature sensors are used to monitor the temperature of the air inside the passenger compartment, as well as the outside air temperature. The climate control system usually has its own separate outside air temperature sensor located outside the engine compartment so engine heat does not affect it. The outside air temperature sensor will usually be mounted behind the grille or in the cowl area at the base of the windshield.). Most of these sensors work exactly the same as the engine air temperature sensor. But some use an infrared sensor to monitor the body temperature of the vehicleís occupants.
CAUSES OF FAILURE
An air temperature sensor can sometimes be damaged by
backfiring in the intake manifold. Carbon and oil contamination inside the intake manifold can also coat the tip of the sensor, making it less responsive to sudden changes in air temperature. The air temperature sensor itself may also degrade as a result of heat or old age, causing it to respond more slowly or not at all.
Sensor problems can also be caused by poor electrical connections at the sensor. A loose or corroded wiring connector can affect the sensorís output, as can damaged wiring in the circuit between the sensor and PCM.
If the intake air temperature sensor is not reading accurately, the PCM may think the air is warmer or colder than it actually is, causing it to miscalculate the air/fuel mixture. The result may be a lean or rich fuel mixture that causes driveability symptoms such as poor idle quality when cold, stumble on cold acceleration, and surging when the engine is warm.
If the engine computer uses the air temperature sensor input to turn on a cold start injector, and the sensor is not reading accurately, it may prevent the cold start injector from working causing a hard cold start condition.
A faulty air temperature sensor may also affect the operation of the EGR valve is the PCM uses air temperature to determine when the EGR valve opens (on most, it uses the coolant temperature input).
On OBD II application (1996 & newer vehicles), a faulty air temperature sensor may prevent the Evaporative (EVAP) Emissions System Monitor from completing. This can prevent a vehicle from passing a plug-in OBD II test (because all the OBD II monitors must run before it can pass the test). The EVAP monitor will only run when the outside temperature is within a certain range (not too cold and not too hot, as a rule).
A faulty air temperature sensor that is reading warmer than normal will typically cause in a lean fuel condition. This increases the risk of detonation and lean misfire (which hurts fuel economy and increases emissions).
A faulty air temperature sensor that is reading colder than normal will typically cause a rich fuel condition. This wastes fuel and also increases emissions.
Sometimes what appears to be a fuel mixture balance problem
due to a faulty air temperature sensor is actually due to
something else, like an engine vacuum leak or even a restricted catalytic converter! A severe exhaust restriction will reduce intake vacuum and airflow causing the sensor to read hotter than normal (because it is picking up heat from the engine).
DIAGNOSING THE AIR TEMPERATURE SENSOR
A faulty air temperature sensor may or may not set a code and turn on the Check Engine light. If the sensor circuit is open or shorted, it will usually set a code. But if it is only reading high or low, or is sluggish due to old age or contamination, it usually will not set a code.
A quick way to check the air temperature sensor is to use a scan tool to compare the air temperature reading to the coolant temperature reading once the engine is warm. A good air temperature sensor will usually read a few degrees cooler
than the coolant sensor.
The sensor's resistance can also be checked with an ohmmeter.
Remove the sensor, then connect the two leads on the ohmmeter to the two pins in or on the sensorís wiring connector plug to measure the sensorís resistance. Measure the sensorís resistance when it is cold. Then blow
hot air at the tip of the sensor with a blow drier (never use a propane torch!) and measure the resistance again. Look for a change in the resistance reading as the sensor warms up.
No change in the sensorís resistance reading as it heats up would tell you the sensor is bad and needs to be replaced. The sensor reading should gradually decrease if the sensor is a negative thermistor, or gradully increase if it is a positive thermistor. If the reading suddenly goes open (infinite resistance) or shorts out (little or no resistance), you have a bad sensor.
To be really accurate, you should look up the resistance specifications for the air temperature sensor, then measure the sensorís resistance at low, mid-range and high temperatures to see if it matches the specifications. A sensor that reads within the specified range when cold, may go out of range at higher temperature, or vice versa. Such a sensor would not be accurate and should be replaced.
The resistance and/or voltage test specifications for the air temperature sensor on your engine can be found in a service manual, or by subscribing to the service information on the (Vehicle Mfrs Service Information Website or AlldataDIY.
The air temperature sensor is a solid state device so no adjustment is possible. However, it may be possible to clean a dirty sensor so that it functions normally once again provided it is still in good working condition. Contaminants can be removed from the tip of the sensor by (1) removing the sensor from the intake manifold, then (2) spraying the sensor tip with electronics cleaner. For sensors that are mounted inside a MAF sensor, the wire sensing element can also be sprayed with aerosol electronics cleaner. Do not use any other type of cleaner as it may damage the plastic housing or leave behind a chemical residue that may cause problems down the road.
If a sensor is not reading within specifications or has failed, replace it. Fortunately, most air temperature are not very expensive (typically less than $30). Dealers always charge more than aftermarket auto parts stores, so shop around and compare prices before you buy. Labor to change an air temperature sensor is usually minimal, unless the sensor is buried under a lot of other stuff that has to be removed.
When replacing the air temperature sensor, be careful not to overtighten it as this may damage the sensor housing, or the threads in a plastic intake manifold.