Ambient Air Temperature Sensor
![Ambient Air Temperature Sensor](graphics/ac-ambient-temperature-sensor.png)
The ambient (ATC) or outside air temperature sensor is an (NTC) negative temperature coefficient sensor that informs the semiautomatic/automatic temperature control system of outside air temperature. The NTC sensor's resistance decreases as the outside air temperature increases. The computer uses this input and other in-car temperature sensors to control temperature and blower speed. When this sensor has a problem, performance suffers, and the compressor's clutch may not engage.
![Bidirectional scan tool.](graphics/scan-tool-1.jpg)
Diagnose ambient air temperature sensor symptoms, like an inoperative clutch or input problems, with a scan tool and a multimeter. Factory and many generic scan tools can activate specific components; the compressor clutch is one of them. If the clutch engages with the scan tool, there is likely a problem with one of the inputs. Verify any suspicions with a visual inspection of the sensor and its connector. To test this sensor for resistance, use a multimeter and verify the manufacturer's specifications (~ 220-240 ohms at 70°-80° F).
![Automotive air conditioning NTC negative coefficient sensors.](graphics/ac-ntc-sensors.png)
Automotive air conditioning systems use NTC sensors to monitor air temperature. The In-Car Temperature Sensor (ICTS) is an NTC that monitors the air temperature of the passenger compartment.
![AC Class B climate control network.](graphics/diagram-ac-climate-control-can-ex.png)
The ambient temperature sensor (ATS) monitors the air temperature outside the vehicle. The EATC compares these values and makes output decisions based on the difference between these air temperatures.