SPN 102 - FMI 4 (Fault Code 123)

SPN 102 - FMI 4 (Fault Code: 123)
ISB, ISC, and ISL - Intake Manifold 1 Pressure Sensor

Fault Code: 123
PID(P), SID(S): P102
SPN: 102
FMI: 4/4

REASON: Intake Manifold 1 Pressure Sensor Circuit - Voltage Below Normal or Shorted to Low Source. Low signal voltage or open circuit detected at the intake manifold pressure circuit.

EFFECT: Derate in power output of the engine.

Circuit Description:
The electronic control module (ECM) provides a 5 volt supply to the intake manifold pressure sensor on the sensor supply circuit. The ECM also provides a ground on the sensor return circuit. The intake manifold pressure sensor provides a signal to the ECM on the intake manifold pressure sensor signal circuit. This sensor signal voltage changes based on the pressure in the intake manifold. The ECM will detect a low signal voltage at operating conditions such as during an idle or a deceleration. The ECM will detect a high signal voltage during high engine load operating conditions.

Component Location:
The intake manifold pressure sensor is located in the air intake horn.

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 intake manifold pressure signal voltage is less than 0.26-VDC for more than 1 second.

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 intake manifold 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 electronic control module (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 intake manifold 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 123, check for multiple faults.

Possible causes of this fault code include:
-Signal circuit open or shorted to ground in the engine harness or sensor.
-Supply line open or shorted to ground.

Read More:
SPN 102 - FMI 3 (Fault Code 122)
SPN 612 - FMI 2 (Fault Code 115)


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