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SPN 84 FMI 2 — Vehicle Speed Sensor Erratic
CautionElectricalLimp to the next safe stop
Applies to: All trucks
Vehicle Speed Sensor Erratic.
Don't risk it — get a diesel tech on the truck.
Look up listed providers near you and give them this SPN/FMI when you call.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Damaged speed sensor
- improper air gap
- tone wheel damage
- TCM/ECM conflict
- wiring issue
FAQ
What does SPN 84 FMI 2 mean?
Vehicle Speed Sensor Erratic.
How serious is SPN 84 FMI 2?
Caution — Limp to the next safe stop.
What's the most likely cause of SPN 84 FMI 2?
The most common cause is: Damaged speed sensor. Other possibilities include improper air gap; tone wheel damage; TCM/ECM conflict; wiring issue.
Can I keep driving with SPN 84 FMI 2?
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Throwing this code right now?
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.