Case IH 3986 – Engine Control Unit (ECU): Exhaust gas pressure sensor voltage is lower than expected.

The signal voltage from the exhaust gas pressure sensor has dropped below the minimum acceptable threshold. This typically points to a short to ground in the wiring, a disconnected sensor, or an internal failure of the sensor itself.

• Steiger T4B
• Case IH 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Case N-Series Loader Backhoes
• Case IH, New Holland, and Steyr agricultural machinery utilizing FPT engines (F5 or Cursor series)

1. 1

   Inspect the wiring harness leading to the exhaust gas pressure sensor for chafing, melting, or disconnected wires

2. 2

   Disconnect the 3-pin connector at the sensor and check for a proper 5V reference signal and a good ground from the ECU

3. 3

   Test the signal wire for a short to ground, which is the primary cause of a voltage lower than expected code

4. 4

   Inspect the signal tube/hose connecting the sensor to the exhaust system for soot blockages, cracks, or leaks

5. 5

   If the wiring harness tests good (no shorts, proper reference voltage), verify if the sensor has failed internally

1. 1

   Preparation

   Turn off the engine, remove the key, and disconnect the battery to prevent electrical spikes. Allow the exhaust system to cool completely.

2. 2

   Locate the Sensor

   Locate the exhaust gas pressure sensor, which is typically mounted near the DPF/SCR assembly and connected to the exhaust via a metal signal tube.

3. 3

   Disconnect Electrical

   Depress the locking tab and unplug the 3-pin electrical connector from the sensor.

4. 4

   Remove the Sensor

   Carefully loosen and disconnect the exhaust gas pressure sensor signal tube from the sensor. Remove the mounting bolts securing the sensor bracket.

5. 5

   Install the New Sensor

   Mount the new sensor and tighten the retaining bolts.

6. 6

   Reconnect Lines

   Reattach the exhaust gas pressure sensor signal tube. Note: If the EGR flex pipe or related exhaust components were removed for access, OEM manuals mandate replacing the gaskets.

7. 7

   Reconnect Electrical

   Plug the 3-pin connector back into the new sensor until it clicks securely.

8. 8

   Clear Codes

   Reconnect the battery. Use a diagnostic scan tool to clear the inactive DTC 2858 / 3986 code from the Engine ECU. Run the engine to verify the code does not return and that the SCR system functions normally.