Cummins ISB4.5 CM2150 Fault Code: 2398 PID: SPN: 171 FMI: 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 engine 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 troubleshooting and repair 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 eight 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 that the engine intake manifold temperature is higher or lower than the other temperature sensors on the engine after an extended period of engine off time, or the ambient 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
-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. If the fault code does not go inactive, it may be necessary to perform an 8 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 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.

Possible Causes
-Stuck in-range ambient air temperature sensor.
-High resistance in the ambient air temperature sensor SIGNAL or RETURN wires.
-ECM malfunction.