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SPN 84 FMI 8

Wheel-Based Vehicle Speed — Abnormal frequency, pulse width, or period.
A frequency- or PWM-type signal is outside the expected range or shape.

What the SPN covers: Vehicle road speed calculated from a wheel or output-shaft speed signal. More on SPN 84.

What FMI 8 usually points to: Damaged speed sensor or tone ring, wrong air gap, electrical noise, or a failing PWM sensor. More on FMI 8.

Severity

MEDIUM — fix soon

Speedometer, cruise and speed limiting may misbehave. Safe to move carefully to a shop.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Damaged or dirty vehicle speed sensor
  3. Wrong sensor air gap or damaged tone ring
  4. Wiring damage
  5. Incorrect tire size or axle ratio programmed

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Inspect the speed sensor and tone ring for debris and damage
  4. Measure sensor resistance and AC output while turning
  5. Check the parameter programming for tire revs/mile and axle ratio
  6. Look for ABS codes that point to the same sensor

FAQ

What does SPN 84 FMI 8 mean?

Wheel-Based Vehicle Speed — abnormal frequency, pulse width, or period. A frequency- or PWM-type signal is outside the expected range or shape.

Is SPN 84 FMI 8 serious?

Speedometer, cruise and speed limiting may misbehave. Safe to move carefully to a shop.

What should I check first for SPN 84 FMI 8?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Inspect the speed sensor and tone ring for debris and damage.