ALL ABOUT AIR TEMPERATURE SENSORS
Air temperature sensors go by a variety of names. Ford has
Air Charge Temperature (ACT), Vane Air Temperature (VAT) and
Manifold Charging Temperature (MCT) sensors, General Motors has
Manifold Air Temperature (MAT) sensors, Chrysler has Charge
Temperature Sensors (CTS) and Bosch has Air Temperature Sensors
Though all these sensors are basically the same in design and
function as a coolant sensor, air temperature sensors are used in
vehicles equipped with fuel injection for one of two purposes.
On some systems, the sensor is used to monitor the temperature of
the air entering the engine while the engine is cold. The sensor
is mounted in the intake manifold with the tip exposed to air.
It gives a much faster response to temperature changes than the
coolant sensor. If the air is cold enough, the sensor triggers
the cold start injector to spray additional fuel into the
manifold to richen the fuel mixture. In this way it acts
something like an electronic choke while the engine is cold. On
some applications, the air temperature sensor is also used to delay
the opening of the EGR valve until the engine warms up.
In other applications the sensor is used to continuously
monitor the incoming air temperature so the engine computer can
alter the fuel mixture to compensate for changes in air density.
Cold air is denser than warm air, so when the sensor reads cold
injector duration is increased to maintain a balanced fuel
mixture. If the sensor reads hot, then the duration of the
injectors is shortened to prevent the mixture from becoming too
All air temperature sensors work like coolant sensors. Their
electrical resistance changes in direct response to temperature.
When the computer applies a control voltage to the sensor, the
amount of resistance encountered determines the return signal
voltage. When the sensor is cold, resistance is high and the
return signal to the computer is high. As the temperature rises,
resistance drops and the voltage output goes down.
An air temperature sensor can sometimes be damaged by
backfiring in the intake manifold. Carbon and oil contamination
can also affect the accuracy and responsiveness of the sensor as
can old age. Driveability symptoms for the type that are used to
trigger the cold start injector include stalling or rough idle
when cold. A faulty sensor may also allow the EGR valve on some
applications to open before the engine is warm, resulting in cold
hesitation or detonation.
On applications where air temperature is monitored
continuously to help balance the fuel mixture, a faulty sensor
can contribute to cold stumbling and warm surging.
Sometimes what appears to be a fuel mixture balance problem
due to a faulty air temperature sensor is in fact due to
something else, like a restricted catalytic converter! A severe
exhaust restriction will reduce intake vacuum and airflow causing
the sensor to read hotter than normal. Once the engine is warm,
the air temperature sensor will usually read a few degrees cooler
than the coolant sensor.
Trouble codes that may indicate an air temperature sensor
* General Motors: Codes 23 & 25
* Ford: Codes 24, 54 & 64
* Chrysler: Code 23
An air temperature sensor can be checked several ways. On GM
applications, the easiest way is to tap into the onboard
electronics with a scan tool. Plugging into the ALDL connector
will allow you to get a direct temperature reading (in degrees
Celsius) from the sensor. You should see a slight drop in the
temperature reading when the throttle is opened while the engine
is running and air rushes into the manifold. You can also
compare the air temperature and coolant sensor readings to see if
they're close on a warm engine.
The sensor's resistance can also be checked with an ohmmeter.
Remove the sensor and check the resistance when cold. Then blow
hot air at the tip with a blow drier (never use a propane torch!)
and watch for a drop in resistance. No change means a new sensor
All you can do with a defective sensor is replace it (unless
the tip is coated with carbon, in which case cleaning the tip in
solvent may restore its function). Be careful not to overtighten
the sensor when it is installed, and use sealer on the threads to
prevent vacuum leaks.