Cummins ISC CM2150 (2007-09) Fault Code: 256 PID: 171 SPN: 171 FMI: 4 Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal or Shorted to Low Source. Low signal voltage detected at the ambient air temperature circuit.

Verify the electronic control module (ECM) calibration is correct. Check the calibration revision history found on QuickServe® Online for applicable fixes to the calibration stored in the ECM. If necessary, recalibrate the ECM.

The ambient air temperature sensor shares return wires in the engine harness with other sensors. Opens and shorts in the engine harness can cause multiple fault codes to be active. Check fault codes with multiple counts first.

Possible causes:

1)Signal circuit shorted to ground in the harness
2)Signal circuit shorted to return or ground in the sensor.

Possible repairs:

1)Replace the ambient air temperature sensor.
2)Repair or replace the engine harness.
3)Replace the ECM.