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SPN 91 FMI 4

Accelerator Pedal Position 1 — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

What the SPN covers: How far the driver is pressing the throttle pedal, sent as a percentage to the engine computer. More on SPN 91.

What FMI 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

Severity

HIGH — limit use, diagnose now

The engine may go to idle-only or limp mode. Do not drive in traffic with an unreliable throttle.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Worn or failed pedal position sensor
  3. Corroded pedal connector (common under the cab floor)
  4. Chafed harness
  5. Lost 5 V reference shared with other sensors

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. Watch pedal % on a scan tool while slowly pressing — it should rise smoothly
  4. Inspect the pedal connector for water and corrosion
  5. Check the 5 V reference and ground at the pedal
  6. Check if idle validation switch codes are also present

FAQ

What does SPN 91 FMI 4 mean?

Accelerator Pedal Position 1 — voltage below normal or shorted low. The sensor circuit voltage is lower than it should ever be, usually near zero.

Is SPN 91 FMI 4 serious?

The engine may go to idle-only or limp mode. Do not drive in traffic with an unreliable throttle.

What should I check first for SPN 91 FMI 4?

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; Watch pedal % on a scan tool while slowly pressing — it should rise smoothly.