What is SPN 5491 FMI 4 & How to Solve It?

What is SPN 5491 FMI 4:

SPN 5491 refers to the Aftertreatment Diesel Exhaust Fluid (DEF) Pump Relay, and FMI 4 indicates the circuit is shorted to ground or below expected voltage.

This means the ECM is commanding the pump, but it’s not responding due to an electrical fault.

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Why This Happens

• Grounded or shorted DEF pump wiring.
  
• Failed relay or blown fuse.
  
• DEF pump drawing excessive current.
  
• Corroded or damaged relay terminals.
  

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How to Fix It

1. Inspect the DEF pump wiring and test continuity.
2. Check relay voltage and resistance using a multimeter.
3. Replace the relay if it fails electrical tests.
4. Replace the pump if it is causing short-circuit conditions.
5. Clear the fault and test DEF pump activation.

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How to Avoid It Before It Happens

1. Test pump and relay performance regularly.
2. Keep connectors dry and corrosion-free.
3. Replace high-cycle relays before failure.
4. Monitor DEF pressure and flow data during operation.