What is SPN 4377 FMI 12 & How to Solve It?

What is SPN 4377 FMI 12:

SPN 4377 refers to the Diesel Exhaust Fluid (DEF) Dosing Valve, and FMI 12 indicates that the device or component is failing to operate.

Together, SPN 4377 FMI 12 means that the DEF dosing valve is not responding to commands from the engine control module. This prevents the proper injection of DEF into the aftertreatment system, which can trigger derates and increase NOx emissions if left unresolved.

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Why This Happens

The code is typically caused by:

• A failed DEF dosing valve (stuck closed or unresponsive).
• Electrical failure in the valve’s solenoid or motor.
• Open or shorted wiring harness to the valve.
• Faulty DEF controller or signal miscommunication.
• Contamination or crystallization blocking valve movement.

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How to Fix It

Follow these steps to troubleshoot:

1. Inspect the DEF dosing valve for visible damage or crystal buildup.
2. Unplug and clean the valve connector and check for corrosion.
3. Check the wiring harness for continuity and proper voltage.
4. If power is present and wiring is intact, replace the dosing valve.
5. Run diagnostics to confirm if the control unit is sending proper signals.
6. Clear the code and verify if dosing resumes during operation.

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How to Avoid It Before It Happens

1. Use high-quality DEF to reduce crystallization buildup.
2. Schedule regular inspections of the dosing system and its connectors.
3. Keep the DEF tank closed and sealed to avoid contamination.
4. Train your crew to notice signs of irregular exhaust behavior early.