SPN 108 - FMI 3 (Fault Code 221)

SPN 108 - FMI 3 (Fault Code: 221)
ISB - Barometric Pressure Sensor Circuit

Fault Code: 221
PID(P), SID(S): P108
SPN: 108
FMI: 3/3

ISC and ISL - Barometric Pressure Sensor Circuit

Engine power derate.

REASON : Barometric Pressure Sensor Circuit - Voltage Above Normal or Shorted to High Source. High signal voltage detected at barometric pressure circuit.

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

The engine control module (ECM) provides a 5 volt supply to the ambient air pressure sensor on the sensor supply 1 circuit. The ECM also provides a ground on the sensor 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.

Component Location:

  • ISB: The combination ambient air temperature/pressure sensor is located in the original equipment manufacturer (OEM) intake air plumbing.
  • ISC and ISL: The barometric pressure sensor is located near the intake manifold.

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 16 seconds.

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 of the ambient pressure reading is used.

Conditions for Clearing the Fault Code:
The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.

Shop Talk:

  • Verify the ECM calibration is correct. Check the calibration revision history found on QuickServe® Online for applicable fixes to the calibration stored in the ECM. If necessary, recalibrate the ECM.
  • 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 faults.

Possible causes of this fault code include:
• Open return circuit in the harness, connectors, or sensor
• Signal circuit shorted to sensor supply or battery voltage.

Read More:
SPN 105 - FMI 4 (Fault Code 154)
SPN 111 - FMI 18 (Fault Code 197)


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