What is SPN 3520 FMI 1 & How to Solve It?

What is SPN 3520 FMI 1:

SPN 3520 refers to the Aftertreatment 1 DEF Dosing Unit Temperature, and FMI 1 indicates the sensor is reporting a value below normal operating range.

This usually means the DEF dosing unit is too cold, potentially frozen, which can lead to interrupted DEF injection or failed regen.

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Why This Happens

• DEF system exposed to freezing temperatures.
  
• DEF heater not operating properly.
  
• Temperature sensor failed or reading inaccurately.
  
• Cold ambient start with no pre-warming.
  

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How to Fix It

• Move equipment into a warm area and allow the DEF system to thaw.
• Inspect the DEF dosing unit heater and sensor.
• Test sensor values during warm-up and replace if stuck.
• Check DEF quality and storage practices.
• Clear the code and verify dosing resumes normally.
  
  

How to Avoid It Before It Happens

• Test DEF heaters and sensors before cold weather begins.
  
• Store machines in environments above freezing.
  
• Use insulated DEF tanks or heaters for cold regions.
  
• Monitor DEF system warm-up behavior closely.