Detailed guide for SPN 84 FMI 2
Back to top ↑What SPN 84 FMI 2 means
SPN 84 identifies wheel-based vehicle speed.
FMI 2 indicates that the vehicle-speed data is erratic, intermittent or incorrect.
This is not an engine crankshaft-speed fault. Engine speed is represented by a different SPN.
What you may notice
The vehicle-speed value displayed by diagnostic software may jump, disappear or disagree with other speed data.
Cruise control, speed limiting or other functions that depend on reliable vehicle-speed information may be affected.
What to check first
Check for related ABS, wheel-speed or J1939 communication faults.
Inspect the vehicle-speed sensor circuit and associated connectors for loose terminals, damaged wiring or intermittent connections.
Compare the wheel-based vehicle-speed value reported by different controllers. If one controller reports implausible data, follow the OEM diagnostic path for the source of that signal.
Important
The source of SPN 84 data can differ by vehicle design. Some vehicles derive it from wheel-speed information, while others may use drivetrain or tailshaft speed.
Use the vehicle manufacturer's wiring and network documentation before replacing sensors or controllers.
Mechanic community notes for SPN 84 FMI 2
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.