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Cummins ISM CM875 (2003-06) Fault Code: 256 PID: P171 SPN: 171 FMI: 4/4 Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal or Shorted to Low Source. Low voltage detected at ambient air temperature circuit.

Possible causes:

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

Possible repair:

1)Replace the OEM harness.
2)Replace the ambient air temperature sensor.
3)Replace the ECM.