SPN 84 FMI 2 — Vehicle Speed Sensor Erratic
What it means: Vehicle Speed Sensor Erratic.
How serious / can you keep driving: Caution. Limp to the next safe stop.
Check first: Start with the most likely cause: Damaged speed sensor.
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Indianapolis, IN
Closest first. Tell them SPN 84 FMI 2 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Indianapolis, IN · (317) 789-5002
- 0.0 mi away — based in Indianapolis, IN · (317) 300-7000
- 0.0 mi away — based in Indianapolis, IN · (317) 646-8208
Applies to: All trucks
What SPN 84 FMI 2 means
Vehicle Speed Sensor Erratic.
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
Vehicle Speed Sensor Erratic.
Caution — Limp to the next safe stop.
The most common cause is: Damaged speed sensor. Other possibilities include improper air gap; tone wheel damage; TCM/ECM conflict; wiring issue.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside SPN 84 FMI 2, or share the same subsystem.
- Vehicle Speed Signal — Erratic / Lost (SPN 84 FMI 19)
- Service brake air pressure — primary circuit low (SPN 519 FMI 4)
- Service brake air pressure — secondary circuit low (SPN 520 FMI 4)
- Left front wheel speed sensor erratic (SPN 789 FMI 2)
- Left front wheel speed abnormal signal (SPN 789 FMI 8)
- Right front wheel speed sensor erratic (SPN 790 FMI 2)
- Right front wheel speed abnormal (SPN 790 FMI 8)
- Left rear (LF drive) wheel speed erratic (SPN 791 FMI 2)
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.