Freightliner · Accelerator Pedal Position SensorHigh

Freightliner SPN 91 FMI 2: Erratic Accelerator Pedal Signal

SPN 91 FMI 2 (FMI 2) — Accelerator Pedal Position 1 - Data Erratic, Intermittent, or Incorrect

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Quick Answer

SPN 91 FMI 2 means the accelerator pedal signal is jumping erratically rather than sweeping smoothly, usually from a connector or wiring issue at the pedal assembly.

Overview: Freightliner SPN 91 FMI 2

Because throttle input is safety-critical, the ECM reacts by defaulting to idle. The fix is almost always electrical, not a full pedal assembly failure.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Beginner

Parts Cost

$150 - $400

Dealer Labor

$350+

Tools Required

  • Scan Tool
  • Multimeter
  • Electrical Contact Cleaner

What SPN 91 FMI 2 Means

SPN 91 FMI 2 on Freightliner Cascadia and other models fires when the accelerator pedal position signal jumps erratically instead of sweeping smoothly. Since the pedal uses redundant sensor channels, the ECM cross-checks both and flags disagreement or electrical noise on either one, usually dropping the engine to idle for safety.

Symptoms Drivers Notice

  • Engine defaults to idle or a fixed low-power state
  • Surging or hesitant throttle response before the fault fully sets
  • Amber or red stop lamp depending on severity
  • May be intermittent, worsening at certain pedal angles

Most Likely Root Causes

Corroded or loose 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 the same point over time.

Worn or cracked pedal potentiometer

Occasional

Mechanical wear creates electrical noise as the pedal moves.

Common Misdiagnoses (Don't Waste Parts)

Replacing the whole pedal assembly immediately

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

Do this instead: Wiggle-test the connector and harness first - many cases resolve with cleaning or reseating.

Assuming the ECM is at fault

Why it happens: Erratic behavior feels like a control module issue.

Do this instead: Cross-check both redundant pedal channels - a single faulted channel points to wiring.

Master Tech Commentary

Mechanic's Notes

Shop tip: on Cascadias, check the connector at the pedal bracket first - it's a common spot for corrosion given its low position and exposure to floor mat debris and moisture.

Step-by-Step Diagnostic Procedure

  1. Step 1: Inspect the pedal assembly connector under the dash for corrosion or a partially unseated pin.
  2. Step 2: Wiggle-test the harness near the pedal bracket while watching live pedal position data for both channels.
  3. Step 3: Check for chafing where the harness routes near the floor bracket or firewall grommet.
  4. Step 4: Compare both accelerator pedal sensor channels - a channel jumping independently isolates that circuit.
  5. Step 5: Inspect the pedal mechanism for a cracked housing or worn pivot causing mechanical noise in the signal.

Live Data & Freeze-Frame Tips

Watch both accelerator pedal channels while slowly pressing the pedal through its full range - they should move together smoothly with no dropouts.

Safety Warnings

  • Do not drive if throttle behaves unpredictably at speed.
  • Verify the pedal returns fully to idle before test-driving after any repair.

When to Limp In vs Tow

If the engine surges unexpectedly or won't respond predictably, stop and tow rather than risk unintended acceleration.

Prevention Tips for Fleets & Owner-Operators

  • Keep the pedal connector protected from underfloor moisture.
  • Inspect the harness during routine PM services.
  • Address intermittent throttle hesitation early.

Related Fault Codes

SPN 91 FMI 8

Signal missing entirely rather than erratic, usually a harder open-circuit fault.

Frequently Asked Questions

Why does the truck go to idle with this code?

The ECM can't trust an erratic pedal signal for safety and defaults to idle.

Is the pedal assembly usually bad?

Not usually - connector corrosion and harness chafing cause most of these faults.

Can I test this myself?

Yes, with a scan tool showing live pedal channel data and a basic wiggle test.

How urgent is this repair?

Fairly urgent since it directly limits drivability to idle.

Will cleaning the connector fix it?

Often yes, especially if corrosion is the root cause.