Mechanic Glossary

Ambient Air Temperature Sensor

An electrical thermistor sensor mounted near the front grille that measures outside air temperature for the HVAC and engine computer.

The ambient air temperature (AAT) sensor reads the outside air temperature. Mounted behind the front bumper grille away from engine heat, this small thermistor changes its electrical resistance based on temperature. The engine and climate control computers read this data. It displays the temperature on your dashboard clock, and helps the automatic A/C system adjust fan speed and cabin temperature settings.

This sensor is vulnerable to front-end accidents and road debris. If the sensor is damaged or disconnected, the dashboard display may read extremely cold (like -40F) or show a blank line. This can confuse the climate control system, making it refuse to turn on the A/C compressor because it thinks the outside air is already freezing.

Frequently Asked Questions

It is almost always clipped behind the front bumper grille, in front of the radiator, or inside one of the side mirror housings.
When parked, engine heat rises and gets trapped behind the grille. The sensor will read high until you drive for a few minutes, allowing fresh air to cool it back to ambient temperature.
If the sensor fails open, the computer reads -40F. To prevent the evaporator core from freezing, the computer disables the A/C compressor. Replacing the sensor fixes the A/C.
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