Detailed guide for SPN 84 FMI 4
Back to top ↑What SPN 84 FMI 4 means
SPN 84 identifies wheel-based vehicle speed.
FMI 4 indicates voltage below normal or a signal circuit shorted toward a low source.
This is not an engine-speed or crankshaft-sensor code.
What you may notice
The vehicle-speed reading can become unavailable, remain at zero or behave incorrectly.
Functions that depend on vehicle-speed information, such as cruise control or speed limiting, may be disabled on some applications.
What to check first
Inspect the vehicle-speed sensor wiring and connectors for chafing, corrosion, damaged terminals or a short to ground.
Use the OEM wiring diagram to test supply, ground and signal circuits.
Check for related ABS or J1939 faults because wheel-based speed information may originate from another controller.
Important
The exact sensor and signal source depends on vehicle architecture. Diagnose the source controller and wiring before replacing a speed sensor.
Mechanic community notes for SPN 84 FMI 4
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