CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 123

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Overview

ISX – Intake Manifold 1 Pressure Sensor SMALL | MEDIUM | LARGE

ISM – Intake Manifold 1 Pressure Sensor SMALL | MEDIUM | LARGE

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 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. Refer to Procedure 100-002 in Section E.

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, calibrate the ECM. Refer to Procedure 019-032 in Section 19.

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.

Refer to Troubleshooting Fault Code t05-123