Cummins ISB4.5 CM2150 Fault Code: 221 PID: 108 SPN: 108 FMI: 3 Barometric Pressure Sensor Circuit – Voltage Above Normal or Shorted to High Source.

Circuit Description
The ambient air pressure sensor responds to pressure changes in the atmospheric pressure. The pressure changes occur based on elevation that the engine is presently operating at. The ambient air pressure sensor has a 5 volt supply circuit, a sensor signal voltage, and a return circuit.

The ambient air pressure sensor provides a signal to the ECM on the ambient air pressure sensor signal circuit. This sensor signal voltage changes based on the pressure of the atmosphere. The ECM will detect a low signal voltage when the vehicle is operating at high altitudes. The ECM will detect a high signal voltage when the vehicle is operating at sea level altitudes.

Component Location
The barometric pressure sensor is mounted on the main branch of the engine harness near the ECM.

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 pressure signal voltage is greater than 4.75-VDC for more than 10 seconds.

Action Taken When the Fault Code is Active
-The ECM illuminates the amber CHECK ENGINE lamp and/or the Malfunction Indicator Lamp (MIL) immediately, when the diagnostic runs and fails.
-Engine torque could possibly be reduced.
-A default value of the ambient pressure reading is used.

Conditions for Clearing the Fault Code
-To validate the repair, start the engine and let it idle for 1 minute.

For engines equipped with SCR aftertreatment:

-This fault code can not be cleared with INSITE™ electronic service tool.
-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.

For all other engines:

-The ECM will turn off the amber CHECK ENGINE lamp after the diagnostic runs and passes.
-For On-Board Diagnostics (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 fault codes, as well as extinguish the MIL for OBD applications.

Shop Talk
The barometric pressure sensor shares supply and return wires in the engine harness with other sensors. Opens and shorts in the engine harness can cause multiple fault codes to be active. Before troubleshooting Fault Code 221, check for multiple fault codes.

Possible Causes
-Open return circuit in the harness, connectors, or sensor
-Signal circuit shorted to sensor supply or battery voltage.