What is SPN 5798 FMI 10 & How to Solve It?

What is SPN 5798 FMI 10:

SPN 5798 refers to the Aftertreatment 1 DEF Line Heater Relay, and FMI 10 indicates the relay is functioning abnormally, though not completely failed.

This code means that the DEF line heater relay is not operating as expected, which may result in DEF lines freezing during cold conditions. If DEF cannot be delivered properly, it can interrupt the aftertreatment function and cause engine derate.

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Why This Happens

• Relay is sticking or only partially engaging.
• Electrical issue in the relay control circuit.
• Connector corrosion at the relay or heater.
• Intermittent control signal from ECM.
• Internal degradation of the relay due to cycling or heat.

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How to Fix It

1. Locate and test the DEF line heater relay using a multimeter.
2. Check wiring and terminals for corrosion or damage.
3. Verify that the relay is receiving and sending voltage properly.
4. Replace the relay if it is sticking, slow, or overheating.
5. Clear the code and test heater function in cold conditions.

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How to Avoid It Before It Happens

1. Test heater relays before winter operation begins.
2. Inspect connectors and terminals for moisture regularly.
3. Keep the DEF system sealed to prevent water or condensation entry.
4. Replace aging relays during seasonal service.