What is SPN 3509 FMI 4 & How to Solve It?

What is SPN 3509 FMI 4:

SPN 3509 relates to the Aftertreatment Fuel Injector Circuit, and FMI 4 means the circuit is below normal or shorted to low source.

This code appears when there’s a low voltage issue in the circuit controlling DEF or aftertreatment fuel injection, potentially leading to failed regenerations.

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Why This Happens

• Wiring harness shorted to ground.
• Failed injector solenoid pulling voltage down.
• Bad connection or corroded pin at injector plug.
• Control module not supplying proper voltage.
• Broken fuse or circuit interruption.

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How to Fix It

1. Inspect the injector harness for insulation damage or shorting.
2. Test voltage at the injector plug under key-on conditions.
3. Repair or replace corroded or burnt connectors.
4. Replace the aftertreatment injector if it causes consistent shorts.
5. Clear the fault code and test injector operation during regen.

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How to Avoid It Before It Happens

1. Include circuit voltage tests during PM servicing.
2. Use sealed connectors and proper harness routing.
3. Inspect aftertreatment injectors for burn marks or electrical odor.
4. Replace damaged harnesses before full failure.
5. Use Clue to monitor injection system faults and trigger alerts early.