Case IH 3526 – Engine Control Unit (ECU) / Engine Management System (EMS) or VCM (Vehicle Control Module): Downstream NOx sensor internal failure (Short Circuit Error)

The downstream NOx sensor is located directly after the Selective Catalytic Reduction (SCR) catalyst in the exhaust system. It measures the nitrogen oxide pollutants making it through the Diesel Oxidation Catalyst (DOC) and SCR to optimize exhaust fluid (DEF) dosing and fuel economy. This specific code triggers when the system detects an internal short circuit within the sensor itself, meaning the sensor is failing to provide plausible data to the computer.

• Steiger T4B
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Farmall (110A, 120A, 130A, 140A)
• Puma (150, 165, 185, 200, 220 CVT)
• Optum (270, 300 CVT)
• Magnum series
• Case Wheel Loaders (1021G Tier 4B)

1. 1

   Visually inspect the downstream NOx sensor (located directly after the SCR in the tailpipe) and its wiring.

2. 2

   Disconnect the 5-pin connector (AMP MCP 1.2 or Turck Eurofast) and inspect pins for corrosion, moisture, or pushed-out terminals.

3. 3

   Use a multimeter to test the machine-side harness for proper voltage (Power, Ground, CAN High, CAN Low) and check for a short to ground or power. If the harness tests good, the short circuit is internal to the NOx sensor.

4. 4

   Check for accompanying codes (e.g., CAN timeout 3533 or upstream sensor codes). If multiple NOx codes are present, refer to Service Bulletin 2017-100 CIH N SB before replacing parts.

1. 1

   Cool Down

   Turn off the engine and allow the exhaust system (DOC and SCR) to cool completely to avoid severe burns.

2. 2

   Disconnect

   Locate the downstream NOx sensor and disconnect its 5-pin electrical connector from the main engine/rear chassis harness.

3. 3

   Removal

   Using a specialized NOx sensor socket or wrench, carefully unscrew the failed NOx sensor from the exhaust pipe.

4. 4

   Preparation

   Inspect the exhaust bung threads for damage. Apply a high-temperature anti-seize compound to the threads of the new sensor (if it does not come pre-applied from the factory).

5. 5

   Installation

   Thread the new downstream NOx sensor into the exhaust pipe by hand to prevent cross-threading, then tighten to the OEM torque specification.

6. 6

   Reconnect

   Plug the 5-pin connector back into the wiring harness, ensuring the locking tab clicks into place. Secure any loose wiring with high-temp zip ties to prevent it from resting on hot exhaust components.

7. 7

   Clear Codes

   Connect the Case IH Electronic Service Tool (EST) to the machine's diagnostic port, clear the 3526 fault code, and run the engine to verify that the code does not return and that the SCR system is functioning properly.