SPN 84 FMI 2
Wheel-Based Vehicle Speed — Data erratic, intermittent, or incorrect.
The signal jumps around, drops in and out, or disagrees with other information the controller has.
What the SPN covers: Vehicle road speed calculated from a wheel or output-shaft speed signal. More on SPN 84.
What FMI 2 usually points to: Loose or corroded connectors, chafed wiring, a failing sensor, or two redundant sensors that do not agree. More on FMI 2.
Severity
MEDIUM — fix soon
Speedometer, cruise and speed limiting may misbehave. Safe to move carefully to a shop.
Likely causes
- Damaged or dirty vehicle speed sensor
- Wrong sensor air gap or damaged tone ring
- Wiring damage
- Incorrect tire size or axle ratio programmed
Checks & fixes (easiest first)
- Inspect the speed sensor and tone ring for debris and damage
- Measure sensor resistance and AC output while turning
- Check the parameter programming for tire revs/mile and axle ratio
- Look for ABS codes that point to the same sensor
FAQ
What does SPN 84 FMI 2 mean?
Wheel-Based Vehicle Speed — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.
Is SPN 84 FMI 2 serious?
Speedometer, cruise and speed limiting may misbehave. Safe to move carefully to a shop.
What should I check first for SPN 84 FMI 2?
Inspect the speed sensor and tone ring for debris and damage; Measure sensor resistance and AC output while turning; Check the parameter programming for tire revs/mile and axle ratio.