What is SPN 520268 FMI 18 & How to Solve It?

What is SPN 520268 FMI 18:

SPN 520268 FMI 18 indicates an abnormal update rate for the Aftertreatment 1 Outlet NOx Sensor. This means the sensor is not communicating or updating its data at the expected rate. The NOx sensor is essential for measuring nitrogen oxide emissions after the Selective Catalytic Reduction (SCR) system. When it fails or lags, emission controls can’t operate correctly, potentially leading to derate conditions.

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Why This Happens

• Faulty or aging NOx sensor.
  
• Damaged or corroded wiring or connectors.
  
• Communication loss with the ECM.
  
• Software or firmware mismatch.
  
• Excessive heat exposure damaging the sensor.
  
  

How to Fix It

1. Use diagnostics to confirm sensor communication faults.
   
2. Inspect the NOx sensor wiring harness for damage or corrosion.
   
3. Unplug and reconnect the sensor to check for connector issues.
   
4. Replace the NOx sensor if it shows abnormal readings or no update.
   
5. Update ECM software if a communication bug is detected.
   
   

How to Avoid It Before It Happens

• Inspect emission sensor wiring during regular service.
  
• Keep connectors sealed and free of debris or moisture.
  
• Use heat shielding near the exhaust sensors if recommended.
  
• Replace NOx sensors as part of long-term maintenance cycles.
  
• Perform regular diagnostic scans to catch intermittent faults early.
• Use Clue’s Preventive Maintenance to schedule regular inspections of the NOx sensor wiring and connectors to prevent communication issues and ensure proper function.