HighDD15

SPN 91 FMI 2 Freightliner DD15 — Accelerator Pedal Po (2026)

SPN 91 FMI 2 on Freightliner DD15 means accelerator pedal position 1 - data erratic, intermittent, or incorrect. 60–80% of field cases trace to corroded or loose pedal connector. Shop repair with parts runs $150 - $400; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.

What This Code Means

SPN 91 FMI 2 on Freightliner DD15 tells the ECM that accelerator pedal position sensor is outside calibrated limits. On J1939, FMI 2 specifies the failure mode: Accelerator Pedal Position 1 - Data Erratic, Intermittent, or Incorrect. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.

Common Symptoms

• Engine defaults to idle or a fixed low-power state (100% of cases)
• Surging or hesitant throttle response before the fault fully sets (80% of cases)
• Amber or red stop lamp depending on severity (60% of cases)
• May be intermittent, worsening at certain pedal angles (40% of cases)

Likely Causes (Ranked by Probability)

1. Corroded or loose pedal connector (45% of cases) — The low-mounted connector is exposed to moisture and floor debris.

2. Chafed harness near the floor bracket (25% of cases) — Repeated pedal movement flexes the harness at the same point over time.

3. Worn or cracked pedal potentiometer (15% of cases) — Mechanical wear creates electrical noise as the pedal moves.

Step-by-Step Diagnostic Guide

1. Inspect the pedal assembly connector under the dash for corrosion or a partially unseated pin. — Tool: scan tool or multimeter. Document readings on the work order.

2. Wiggle-test the harness near the pedal bracket while watching live pedal position data for both channels. — Tool: scan tool or multimeter. Document readings on the work order.

3. Check for chafing where the harness routes near the floor bracket or firewall grommet. — Tool: scan tool or multimeter. Document readings on the work order.

4. Compare both accelerator pedal sensor channels - a channel jumping independently isolates that circuit. — Tool: scan tool or multimeter. Document readings on the work order.

5. Inspect the pedal mechanism for a cracked housing or worn pivot causing mechanical noise in the signal. — Tool: scan tool or multimeter. Document readings on the work order.

Repair Options (with Costs)

Repair	DIY	Independent Shop	Dealer
Accelerator Pedal Position Sensor wiring repair	$25–$150	$150–$350	$250–$500
Accelerator Pedal Position Sensor sensor/part	$150 - $400	$$150 - $400 + labor	$350+
Full module replacement	N/A	$600–$1,500	$350+

Platform-Specific Notes

On Freightliner DD15, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.

When This Code Sets

SPN 91 FMI 2 sets when accelerator pedal position sensor fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Freightliner captures RPM, torque, exhaust temperature, and DEF level at fault set.

Scan Tool Parameters to Log

• Accelerator Pedal Position Sensor live data PID at idle, cruise, and full load
• Battery voltage and ECM supply voltage
• Related aftertreatment and protection PIDs (SPN 1569 if stacked)

Common Misdiagnoses

Replacing the whole pedal assembly immediately — It’s a straightforward bolt-in fix.. Instead: Wiggle-test the connector and harness first - many cases resolve with cleaning or reseating.

Assuming the ECM is at fault — Erratic behavior feels like a control module issue.. Instead: Cross-check both redundant pedal channels - a single faulted channel points to wiring.

Shop Notes from the Field

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.

Safety Warning

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

Prevention Tips

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

Freightliner Field Notes for SPN 91 FMI 2

• Capture a 20-minute data log of accelerator pedal position sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
• Check for OEM technical service bulletins covering SPN 91 FMI 2 on DD15 — many bulletin fixes address harness routing rather than sensor replacement.
• Torque accelerator pedal position sensor mounting fasteners per Freightliner OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
• Document ambient temperature and key-cycle count at fault set on Freightliner DD15 — cold-soak faults on accelerator pedal position sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 91 FMI 2 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If accelerator pedal position sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Freightliner.
• Capture a 20-minute data log of accelerator pedal position sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
• Check for OEM technical service bulletins covering SPN 91 FMI 2 on DD15 — many bulletin fixes address harness routing rather than sensor replacement.
• Torque accelerator pedal position sensor mounting fasteners per Freightliner OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
• Document ambient temperature and key-cycle count at fault set on Freightliner DD15 — cold-soak faults on accelerator pedal position sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 91 FMI 2 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If accelerator pedal position sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Freightliner.

Related Codes

• SPN 91 FMI 8 — often stacks with SPN 91 FMI 2

FAQ

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.

What does SPN 91 FMI 2 mean on Freightliner DD15?

SPN 91 FMI 2 means accelerator pedal position 1 - data erratic, intermittent, or incorrect. The ECM detected accelerator pedal position sensor outside calibrated limits. Verify with freeze-frame data before clearing.

Can I drive with SPN 91 FMI 2 active?

Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.

How much does it cost to fix SPN 91 FMI 2?

DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $150 - $400. Dealer pricing runs $350+.

What is the most common cause of SPN 91 FMI 2?

Corroded or loose pedal connector — confirm with visual inspection and live data before ordering parts.

Sources

• SAE J1939-71 — SPN 91 parameter definition
• Freightliner OEM service information — verify by engine serial
• Mitchell1 ProDemand — 2026 labor and parts cost surveys

Related Reading

• Freightliner fault codes hub
• Diesel repair cost estimator
• Accelerator Pedal Position Sensor diagnostic guide
• SPN 91 FMI 8