What is SPN 3609 FMI 4 & How to Solve It?

What is SPN 3609 FMI 4:

SPN 3609 refers to the Aftertreatment Fuel Injector, and FMI 4 means the circuit is shorted to ground or voltage is below required levels.

This can prevent fuel delivery for regen cycles.

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Why This Happens

• Shorted wiring harness or connector.
• Injector solenoid failed.
• Open circuit in injector control line.

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How to Fix It

1. Inspect the injector and harness for continuity and short circuits.
2. Replace damaged connectors or wiring.
3. Test solenoid resistance and replace the injector if needed.
4. Clear the code and validate injector activity during regen.

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How to Avoid It Before It Happens

1. Include injector solenoid tests in seasonal inspections.
2. Protect wiring from pinch points and engine heat.