What is SPN 3363 FMI 16 & How to Solve It?

What is SPN 3363 FMI 16:

SPN 3363 FMI 16 means the Diesel Exhaust Fluid (DEF) dosing system is injecting less fluid than required. This low dosing condition prevents the Selective Catalytic Reduction (SCR) system from properly reducing nitrogen oxide (NOx) emissions. The FMI 16 specifically indicates that the DEF dosing rate is below the minimum threshold during normal operation, which can eventually trigger engine derate or emission compliance issues.

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Why This Happens

• Crystallization or clogging in the DEF dosing injector
  
• Low DEF pump pressure or failing pump
  
• Blocked or restricted DEF supply lines
  
• Malfunctioning DEF dosing control module
  
• Contaminated or poor-quality DEF fluid
  
  

How to Fix It

1. Remove and inspect the DEF injector for blockage; clean or replace as needed
   
2. Check DEF fluid quality—replace if it fails a refractometer or visual test
   
3. Test DEF pump pressure using diagnostic tools; replace the pump if weak
   
4. Inspect DEF lines and filters for restrictions; flush or replace if needed
   
5. Run a forced regeneration cycle if available to clear buildup in the SCR
   
   

How to Avoid It Before It Happens

• Always use DEF that meets ISO 22241 standards
  
• Store DEF away from direct sunlight and freezing temperatures
  
• Perform regular DEF system inspections and maintenance
  
• Replace DEF filters as recommended by the manufacturer
  
• Allow regeneration cycles to complete fully to prevent buildup in the exhaust system
• Implement Clue’s Equipment Tracking to monitor DEF fluid quality and dosing levels in real time, helping prevent performance issues with the SCR system.