Cummins X12 CM2350 X119B (2018 & Newer) Fault Code: 7863 PID: SPN: 171 FMI: 10/10 Ambient Air Temperature – Abnormal Rate of Change
The ambient air temperature sensor is reading an erratic value.
Possible reduced engine performance.
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 Engine 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.
The mounting location for the ambient air temperature sensor is OEM dependent.
Conditions for Running the Diagnostic:
The engine must be turned OFF for period of 8 hours before this diagnostic will run. This diagnostic runs when the keyswitch is turned to the ON position after an 8 hour cold soak.
Conditions for Setting the Fault Codes:
The Engine Control Module (ECM) detected the ambient air temperature reading was higher or lower than the other temperature sensors on the engine after an extended period of engine off time, or the ambient air temperature was reading was too high or too low for the engine operating conditions.
Action Taken When the Fault Code is Active:
1. The ECM illuminates the amber check engine lamp and/or the Malfunction Indicator Lamp (MIL) immediately when the diagnostic runs and fails.
2. A default value for the ambient air temperature reading is used.
3. Active and stationary regeneration of the diesel particulate filter will be disabled.
4. Engine torque will be reduced if the engine is operated for an extended period of time with this fault active.
Conditions for Clearing the Fault Code:
1. To validate the repair, the engine must be shut down with the keyswitch in the OFF position for 8 hours. After the 8 hour cold soak turn the keyswitch to the ON position for 30 seconds.
2. The fault code status displayed by the recommended diagnostic tool or equivalent will change to inactive immediately after the diagnostic runs and passes.
3. The ECM will turn off the amber check engine lamp immediately after the diagnostic runs and passes.
4. For On-Board Diagnostics (OBD) engines, the ECM will extinguish the Malfunction Indicator Lamp (MIL) after three consecutive trips where the diagnostic runs and passes.
5. The “reset all faults” command in the recommended diagnostic tool or equivalent can be used to clear active and inactive faults, as well as extinguish the MIL for OBD applications.
Possible causes of this fault code include:
1. Stuck in-range ambient air temperature sensor.
2. High resistance in the ambient air temperature sensor signal or return lines.
3. ECM malfunction.
4. Reference the appropriate OEM wiring diagram when troubleshooting circuits that utilize wiring supplied by the OEM.
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