Case IH F-3220-12 - ECU – Engine Control Unit (ECU): Upstream NOx sensor stability time exceeded / NOx estimation model in ECU software is inaccurate due to other failures

The Engine Control Unit (ECU) uses a mathematical model to estimate the amount of Nitrogen Oxides (NOx) the engine is producing before the exhaust reaches the Selective Catalytic Reduction (SCR) catalyst. This fault indicates that the ECU has disabled or flagged its NOx estimation model as inaccurate because a prerequisite sensor (such as the upstream NOx sensor, ambient air temperature sensor, or humidity sensor) is failing, providing unstable readings, or taking too long to stabilize.

• MAGNUM AFS Connect
• Farmall 110A
• Farmall 120A
• Farmall 130A
• Farmall 140A
• Puma 150
• Puma 165
• Puma 185
• Puma 200
• Puma 220
• Optum 270 CVT
• Optum 300 CVT
• Magnum Series (Tier 4 SCR-equipped)
• Steiger Series (Tier 4 SCR-equipped)

1. 1

   Check for Accompanying Fault Codes: Scan the ECU for other active codes. Look specifically for ambient air temperature sensor faults (e.g., 3517, 3518), humidity sensor faults, or upstream NOx sensor faults (e.g., 3222, 3224, 3225).

2. 2

   Inspect the Upstream NOx Sensor: Check the 5-pin connector and wiring harness leading to the upstream NOx sensor for chafing, melting, or corrosion.

3. 3

   Verify Voltage: Check battery voltage and ECU power supply. Low voltage during cold starts can cause sensor reading instability, triggering stability time-out codes.

4. 4

   Check for Exhaust Leaks: Inspect the exhaust piping between the turbocharger and the SCR catalyst. A leak can introduce unmetered oxygen, skewing the NOx sensor readings and causing the estimation model to fail plausibility checks.

1. 1

   Address Prerequisite Faults

   Diagnose and repair any other active engine or emissions-related fault codes first. If an ambient air temperature or humidity sensor code is present, replace that sensor before touching the NOx system.

2. 2

   Replace the Upstream NOx Sensor

   If no other codes are present (or if upstream NOx stability codes like 3220-12 remain active), the upstream NOx sensor is likely fouled or internally failing.

3. 3

   Remove the Old Sensor

   Disconnect the 5-pin harness. Carefully unthread the NOx sensor probe from the exhaust pipe (use penetrating oil if seized to avoid stripping the exhaust bung threads).

4. 4

   Install the New Sensor

   Thread the new OEM NOx sensor into the exhaust bung and tighten to OEM torque specifications. Route the wiring away from extreme heat sources and reconnect the harness.

5. 5

   Clear Codes and Test

   Use a diagnostic tool (like the CNH Electronic Service Tool) to clear the inactive fault codes.

6. 6

   Perform SCR System Test

   Run a forced stationary regeneration or an SCR system test to allow the ECU to heat the new NOx sensor, verify its stability, and confirm the NOx estimation model is functioning correctly.