Cummins ISB4.5 CM2150 Fault Code: 249 PID: 171 SPN: 171 FMI: 3 Ambient Air Temperature Sensor 1 Circuit – Voltage Above Normal or Shorted to High Source.

Circuit Description
The ambient air temperature sensor is used by the electronic control module (ECM) to monitor ambient air temperature. This can be used to sense the temperature inside the cab of a vehicle or the air temperature outside of the vehicle. The ambient air temperature signal is used by the ECM for the Idle Shutdown Ambient Air Temperature Override feature.

The ambient air temperature sensor is used by the ECM to monitor the ambient air temperature. The ambient air temperature sensor is a thermistor and changes resistance based on the temperature being measured. The ECM provides a 5 volt reference voltage to the sensor. The ECM monitors the change in signal voltage and converts this to a temperature value.

Component Location
The mounting location for the ambient air temperature sensor is OEM dependent. Refer to the appropriate OEM service manual.

Conditions for Running the Diagnostics
This diagnostic runs when the keyswitch is in the ON position and when the PWM device is getting energized or de-energized. In some cases, the diagnostics can also run at certain fixed intervals.

Conditions for Setting the Fault Codes
The fan control circuit PWM signal is not detected to be system voltage when the PWM signal is turned ON.

Action Taken When the Fault Code is Active
-The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
-The fan will not be enabled.

Conditions for Clearing the Fault Code
-To validate the repair, start the engine and let it idle for 1 minute.
– The ECM will turn off the amber CHECK ENGINE light and the fault code will go inactive immediately after the diagnostic runs and passes.
– This fault code can also be cleared with INSITE™ electronic service tool.

Shop Talk
The fan control logic can be adjusted with INSITE™ electronic service tool. There are two choices: 12-VDC or 24-VDC ON, or 0-VDC ON. For proper fan operation, the fan control logic setup with INSITE™ electronic service tool must match the hardware configuration on the vehicle.

This fault code is logged when the ECM commands 12-VDC or 24-VDC to the fan control solenoid, but the voltage on the fan control signal circuit is less than 12-VDC or 24-VDC. The low voltage on the signal circuit usually indicates a short circuit to ground in the harness or solenoid.