Cummins ISX11.9 CM2250 (2010-12) 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. The electronic control module (ECM) uses this temperature reading to optimize engine performance, and determine if heating of the 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 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 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 light immediately when the diagnostic runs and fails.
A default value for the ambient air temperature reading is used.
Conditions for Clearing the Fault Code
To validate the repair, perform a key cycle and leave the key in the ON position for 1 minute.
The fault code status displayed by INSITE? electronic service tool will change to INACTIVE immediately after the diagnostic runs and passes.
The ECM will turn off the amber CHECK ENGINE lamp after the diagnostic runs and passes.
For OBD engines, the ECM will turn off the Malfunction Indicator Lamp (MIL) after 3 consecutive trips where the diagnostic runs and passes.
The ?Reset All Faults? command in INSITE? electronic service tool can be used to clear active and inactive faults, as well as extinguish the MIL for OBD engines.

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 of this fault code include:

Signal shorted to ground in the harness
Signal shorted to return or ground in the sensor.
Reference the appropriate OEM wiring diagram when troubleshooting circuits that utilize wiring supplied by the OEM.

For intermittent power supply and datalink communication issues with Aftertreatment Components, it is highly recommended that the OEM Power Distribution Center fuses and relays be thoroughly checked for loose, missing or intermittent connections.

Possible Cause:
1) OEM harness
2) ECM failure