Kenworth · Accelerator Pedal Position Sensor CircuitHigh

Kenworth SPN 91 FMI 3: Accelerator Pedal Sensor Circuit Shorted High

SPN 91 FMI 3 (FMI 3) — Accelerator Pedal Position 1 - Voltage Above Normal, Circuit Shorted High

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Quick Answer

SPN 91 FMI 3 means the accelerator pedal signal is electrically shorted high, forcing the engine to idle for safety until the wiring fault is fixed.

Overview: Kenworth SPN 91 FMI 3

Throttle-related faults get an aggressive, safety-first response from the ECM. The fix is almost always in the connector or harness at the pedal assembly.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Beginner

Parts Cost

$150 - $400

Dealer Labor

$350+

Tools Required

  • Multimeter
  • Scan Tool
  • Wiring Diagram

What SPN 91 FMI 3 Means

SPN 91 FMI 3 on Kenworth trucks means the accelerator pedal position sensor's signal circuit is reading an electrically impossible high voltage. Because throttle input is safety-critical, the ECM defaults to idle or a fixed low-power state until a valid signal is restored.

Symptoms Drivers Notice

  • Engine limited to idle or a low fixed power state
  • Amber or red warning lamp depending on severity
  • No throttle response from the accelerator pedal
  • Fault may be intermittent if caused by a vibration-related short

Most Likely Root Causes

Pinched or corroded pedal connector

Common

The low-mounted connector is exposed to moisture and floor debris.

Chafed harness near the floor bracket

Common

Repeated pedal movement flexes the harness at a consistent point.

Failed sensor internally shorted

Occasional

Less common than wiring but occurs with age.

Common Misdiagnoses (Don't Waste Parts)

Replacing the pedal assembly immediately

Why it happens: It's a fast bolt-in fix.

Do this instead: Backprobe the wiring first - many cases are connector or harness issues.

Suspecting the ECM

Why it happens: A sudden idle-only state feels like a control module problem.

Do this instead: Cross-check both redundant pedal channels.

Master Tech Commentary

Mechanic's Notes

Shop tip: check the connector at the pedal bracket first on Kenworth T680/W900 cabs - it's a low, exposed location prone to moisture ingress.

Step-by-Step Diagnostic Procedure

  1. Step 1: Inspect the pedal assembly connector for a pinched wire or corrosion at the pins.
  2. Step 2: Backprobe the signal wire with key on, engine off - a voltage pegged near supply voltage confirms the short.
  3. Step 3: Check harness routing near the floor bracket and firewall grommet for chafing.
  4. Step 4: Compare both redundant pedal position channels on a scan tool.
  5. Step 5: Replace the pedal assembly only after wiring has been confirmed clean.

Live Data & Freeze-Frame Tips

Watch both pedal position channels together while pressing the pedal through its range - a channel pinned high confirms the short.

Safety Warnings

  • Do not drive if throttle response is unpredictable.
  • Verify normal pedal return before test-driving after repair.

When to Limp In vs Tow

If the truck won't respond normally to throttle input, tow rather than drive it.

Prevention Tips for Fleets & Owner-Operators

  • Protect the pedal connector from underfloor moisture.
  • Inspect the harness during PM services.
  • Address early throttle hesitation before it becomes a hard fault.

Related Fault Codes

SPN 91 FMI 4 — Accelerator Pedal Position

Same circuit shorted low instead of high.

Frequently Asked Questions

Why does the truck only idle with this code?

The ECM defaults to idle for safety when it can't trust the throttle signal.

Is the pedal assembly usually bad?

Not usually - connector and wiring issues are more common.

Can I test this myself?

Yes, with a multimeter and basic wiring diagram.

How urgent is this repair?

Fairly urgent since it directly limits drivability.

Will a wiring repair fix it?

Yes, in most cases - confirm with backprobing first.

More Kenworth codes on this system

All Kenworth fault codes

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