Detroit Diesel · Accelerator Pedal Position SensorHigh

Detroit Diesel 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 position signal is jumping erratically rather than sweeping smoothly, usually from a connector or wiring issue at the pedal assembly.

Overview: Detroit Diesel SPN 91 FMI 2

Because throttle input is safety-critical, the ECM reacts aggressively to any erratic pedal signal by defaulting to idle. The good news is the fix is almost always electrical and inexpensive - full pedal assembly failure is less common than a bad connection.

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 fires when the ECM sees the accelerator pedal position signal jumping around in ways that don't match a real driver input - rapid, erratic changes rather than a smooth analog sweep. Since the pedal is a redundant dual-sensor design, the ECM cross-checks both channels and flags disagreement or noise on either one.

Symptoms Drivers Notice

  • Engine defaults to idle or a fixed low-power limp mode since the ECM can't trust pedal input
  • Surging or hesitant throttle response before the fault fully sets
  • Amber or red stop lamp depending on severity
  • May be intermittent, worsening when the pedal assembly is pressed at certain angles

Most Likely Root Causes

Corroded or loose pedal connector

Common

The low-mounted connector is exposed to road spray, salt, and moisture over time.

Chafed harness near the floor bracket

Common

Repeated pedal movement flexes the harness at the same point until insulation wears through.

Worn or cracked pedal potentiometer

Occasional

Mechanical wear inside the sensor itself 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 technicians default to.

Do this instead: Wiggle-test the connector and harness while watching live data first - many cases resolve with a simple connector cleaning or reseat.

Assuming the ECM itself is at fault

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

Do this instead: Cross-check both redundant pedal channels - if only one is erratic, it's a sensor/wiring issue, not the ECM.

Master Tech Commentary

Mechanic's Notes

Shop tip: this is one of the more driver-noticeable codes since the truck often goes straight to idle. Check the connector at the pedal bracket first - road salt and moisture love that low-mounted connector.

Step-by-Step Diagnostic Procedure

  1. Step 1: Inspect the pedal assembly connector for corrosion, loose pins, or a partially unseated connector.
  2. Step 2: Wiggle-test the harness near the pedal bracket while watching live pedal position data for both sensor channels.
  3. Step 3: Check for chafing where the pedal harness routes near the floor bracket or firewall grommet.
  4. Step 4: Compare both accelerator pedal sensor channels on a scan tool - they should track each other smoothly; a channel that jumps independently points to that specific circuit.
  5. Step 5: Inspect the pedal mechanism itself for a cracked potentiometer housing or worn pivot causing mechanical noise in the signal.

Live Data & Freeze-Frame Tips

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

Safety Warnings

  • Do not drive if the throttle behaves unpredictably at speed - have it towed or diagnosed before continuing.
  • Verify the pedal returns fully to idle position before test-driving after any repair.

When to Limp In vs Tow

If the engine surges unexpectedly or won't respond predictably to pedal input while driving, stop and have it towed rather than risk unintended acceleration or loss of control.

Prevention Tips for Fleets & Owner-Operators

  • Keep the pedal connector protected from underfloor moisture and road spray.
  • Inspect the harness at the floor bracket during routine PM services.
  • Address any intermittent throttle hesitation early before it becomes a hard fault.

Related Fault Codes

SPN 91 FMI 8

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

SPN 29

Accelerator pedal position 2 (idle validation switch) shares the same connector and can fault together.

Frequently Asked Questions

Why does the truck go to idle with this code?

The ECM can't trust an erratic pedal signal for safety reasons, so it defaults to idle until a valid signal returns.

Is the pedal assembly usually bad?

Not usually on the first occurrence - connector corrosion and harness chafing cause the majority of these faults.

Can I test this myself?

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

How urgent is this repair?

Fairly urgent since it affects drivability directly - most drivers won't tolerate an idle-only engine for long.

Will cleaning the connector really fix it?

Often, yes - especially if corrosion or a loose pin is the root cause rather than a failed sensor.