CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 295

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Overview

ISX – Barometric Pressure Circuit SMALL | MEDIUM | LARGE

ISM – Barometric Pressure Circuit SMALL | MEDIUM | LARGE

Circuit Description

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

The electronic control module (ECM) provides a 5 VDC 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 altitudes.

Component Location

The barometric pressure sensor is mounted on the main branch of the engine harness near the ECM. Refer to Procedure 100-002 (Engine Diagrams) in Section E for a detailed component location view.

Conditions for Running the Diagnostics

This diagnostic runs when the keyswitch is initially turned on before the engine is started.

The diagnostic only runs once per trip when the keyswitch is initially turned on.

Conditions for Setting the Fault Codes

The ECM detects that the ambient air pressure reading is higher or lower than the other absolute pressure sensors on the engine.

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 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. Refer to Procedure 019-032 (ECM Calibration Code) in Section 19 in the corresponding Troubleshooting and Repair Manual for the engine being serviced.

At key-on, before the engine starts, the sensor readings for intake manifold pressure, exhaust gas pressure, and engine oil pressure are compared with the barometric pressure. This fault code occurs if the barometric pressure sensor reading is different from the other three. This check is only done once after the keyswitch is turned on.

Possible causes of this fault code include:

• Stuck in-range barometric pressure sensor reading.
• High resistance in the barometric pressure sensor signal or return lines.

Refer to Troubleshooting Fault Code t05-295