Cummins ISX12 CM2250 (2010-12) Fault Code: 2398 PID: SPN: 171 2 Ambient Air Temperature- Data Erratic, Intermittent, or Incorrect

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 consists of two parts. The first part runs when the keyswitch is turned ON and before engine speed is detected. The ECM must not see a key cycle for a period of ten hours before this part of the diagnostic will run.

The second part runs when the engine is running.

Conditions for Setting the Fault Codes
The ECM detects the ambient air temperature reading is higher or lower than the other temperature sensors on the engine after an extended period of engine off time, or the ambient air temperature is not changing over time.

Action Taken When the Fault Code is Active
The ECM illuminates the amber CHECK ENGINE lamp and/or Malfunction Indicator Lamp (MIL) 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, start the engine and let it idle for 1 minute. If the fault code does not go inactive, it may be necessary to perform a 10 hour cold soak with the key OFF, then turn the keyswitch to the ON position so that the key ON portion of the diagnostic will run.
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 On-Board Diagnostic (OBD) engines, the ECM will turn off the MIL after three 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 applications.

Shop Talk
Possible causes of this fault code include:

Stuck in-range ambient air temperature sensor
High resistance in the ambient air temperature sensor signal or return lines
ECM malfunction.
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