What is SPN 3226 FMI 4 & How to Solve It?

What is SPN 3226 FMI 4:

SPN 3226 FMI 4 indicates a fault in the Diesel Exhaust Fluid (DEF) Dosing Unit Temperature Sensor Circuit, where the sensor is reporting a voltage above the normal range. This means the temperature sensor is detecting a temperature that is higher than expected, which can affect the DEF dosing system's proper operation and emissions control.

Why This Happens

• Faulty DEF dosing unit temperature sensor.
  
• Wiring issues such as damaged, broken, or corroded wires or connectors.
  
• Sensor circuit short to voltage or open circuit.
  
• DEF dosing unit malfunction.
  
• Engine control module malfunction.
  
  

How to Fix It

1. Inspect the DEF dosing unit temperature sensor wiring and connectors for damage or corrosion.
   
2. Test the temperature sensor for proper voltage and resistance.
   
3. Repair or replace any damaged wiring or connectors.
   
4. Replace the DEF dosing unit temperature sensor if faulty.
   
5. Verify the DEF dosing unit functionality and engine control module status.
   
6. Clear the fault code and verify the repair by running the system.
   
   

How to Avoid It Before It Happens

• Regularly inspect and maintain the DEF dosing unit and temperature sensor wiring.
  
• Protect wiring from moisture and physical damage.
  
• Use quality DEF fluid and ensure the system is free of contamination.
  
• Schedule routine diagnostic checks to detect early sensor or wiring faults.
• Use construction software with preventive maintenance features.