Cummins ISB4.5 CM2150 Fault Code: 256 PID: 171 SPN: 171 FMI: 4 Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal or Shorted to Low Source.

Circuit Description
The ambient air temperature sensor is a variable resistor sensor and is used to measure the temperature of the ambient air to determine if heating of the selective catalytic reduction (SCR) system is required. The electronic control module (ECM) supplies 5 volts to the ambient air temperature signal circuit. The ECM monitors the change in voltage caused by changes in the resistance of the sensor to determine the ambient air temperature.

Component Location
The location of the ambient air temperature sensor can vary, and is OEM dependent.

Conditions for Running the Diagnostics
This diagnostic runs continuously when the keyswitch is in the ON position.

Conditions for Setting the Fault Codes
The ECM detects that the ambient air temperature signal voltage is less than 0.12-VDC for more than 1 second.

Action Taken When the Fault Code is Active
-The ECM illuminates the amber CHECK ENGINE lamp and/or the MIL immediately after the diagnostic runs and fails.
-A default value for the ambient air temperature reading is used.
-Engine torque will be reduced.
-The Idle Shutdown Ambient Air Temperature Override feature will use the intake manifold air temperature sensor value to determine idle shutdown and availability of override.

Conditions for Clearing the Fault Code
-This fault code can not be cleared with INSITE™ electronic service tool.
-To validate the repair, start the engine and let it idle for 1 minute.
-The ECM will turn off the amber CHECK ENGINE lamp and/or MIL immediately after the diagnostic runs and passes.
-Use INSITE™ electronic service tool to verify the fault code is inactive.
-The inactive fault code will be cleared from the memory after 9600 hours of engine operation without a reoccurrence.

Shop Talk
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
-Signal shorted to ground in the harness.
-Signal shorted to return or ground in the sensor.