What is SPN 5321 FMI 31 & How to Solve It?

What is SPN 5321 FMI 31:

SPN 5321 FMI 31 indicates a non-responsive or erratic signal from the NOx Sensor Heating Control Circuit. This fault usually means the system has detected invalid or missing data from the heater circuit that warms up the NOx sensor. Proper heating is crucial for the NOx sensor to function accurately, especially during cold starts. Without it, emission monitoring becomes unreliable, and engine derate may follow.

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Why This Happens

• Failed NOx sensor heater element.
  
• Broken or shorted heater circuit wiring.
  
• Poor connection at the sensor or control module.
  
• Blown fuse related to the sensor circuit.
  
• Internal failure in the NOx sensor itself.
  
  

How to Fix It

1. Perform a diagnostic scan to confirm the exact fault.
   
2. Inspect wiring and connectors for damage or corrosion.
   
3. Check the fuse for the sensor heater circuit.
   
4. Test the resistance of the heater element using a multimeter.
   
5. Replace the NOx sensor if the heater is faulty or open.
   
   

How to Avoid It Before It Happens

• Regularly inspect emission sensor wiring and fuses.
  
• Use only OEM-quality NOx sensors.
  
• Avoid moisture exposure near connectors.
  
• Replace sensors at OEM-recommended intervals.
  
• Monitor sensor heater activity in diagnostic reports during service.
• Use Clue’s Equipment Tracking to monitor the NOx sensor heater circuit in real-time, detecting any irregularities or faults before they affect emissions monitoring.