Detailed guide for SPN 5491 FMI 3
Back to top ↑What SPN 5491 FMI 3 means
SPN 5491 FMI 3 is used on documented Navistar and Cummins applications for an aftertreatment DEF line heater relay circuit voltage-high condition.
FMI 3 indicates voltage above normal or a circuit shorted toward a high source.
What you may notice
A DEF heater or SCR-related warning may appear.
Some applications may reduce engine performance if the DEF heating system cannot be controlled correctly.
What to check first
Inspect the DEF line heater relay and relay socket for loose terminals, corrosion, water intrusion or heat damage.
Check the relay control circuit and OEM harness for a short to battery voltage.
Inspect related fuses, power-distribution connections and connectors for intermittent contact.
Important
The relay location and exact circuit layout are vehicle-manufacturer dependent. Use the correct OEM wiring diagram before replacing the relay or aftertreatment controller.
Mechanic community notes for SPN 5491 FMI 3
Back to top ↑Real-world tips from technicians. Submissions are moderated to keep spam and “my cousin fixed it with duct tape” content out.
Share your fix / advice
Keep it useful: symptoms, what you checked, what fixed it, and whether the code stayed inactive after a drive cycle.