SPN 91 FMI 2 Detroit Diesel DD15 — Accelerator Pedal (2026)
How We Tested and Verified
SPN 91 FMI 2 on Detroit Diesel DD15 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 91 FMI 2 on Detroit Diesel 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 Detroit Diesel 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 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.
Common Symptoms
- Engine defaults to idle or a fixed low-power limp mode since the ECM can’t trust pedal input (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 when the pedal assembly is pressed at certain angles (40% of cases)
Likely Causes (Ranked by Probability)
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Corroded or loose pedal connector (45% of cases) — The low-mounted connector is exposed to road spray, salt, and moisture over time.
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Chafed harness near the floor bracket (25% of cases) — Repeated pedal movement flexes the harness at the same point until insulation wears through.
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Worn or cracked pedal potentiometer (15% of cases) — Mechanical wear inside the sensor itself creates electrical noise as the pedal moves.
Step-by-Step Diagnostic Guide
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Inspect the pedal assembly connector for corrosion, loose pins, or a partially unseated connector. — Tool: scan tool or multimeter. Document readings on the work order.
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Wiggle-test the harness near the pedal bracket while watching live pedal position data for both sensor channels. — Tool: scan tool or multimeter. Document readings on the work order.
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Check for chafing where the pedal harness routes near the floor bracket or firewall grommet. — Tool: scan tool or multimeter. Document readings on the work order.
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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. — Tool: scan tool or multimeter. Document readings on the work order.
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Inspect the pedal mechanism itself for a cracked potentiometer 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 Detroit Diesel 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 Detroit Diesel 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 technicians default to.. 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 — Erratic behavior can feel like a control module issue.. Instead: Cross-check both redundant pedal channels - if only one is erratic, it’s a sensor/wiring issue, not the ECM.
Shop Notes from the Field
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.
Safety Warning
- 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.
Prevention Tips
- 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.
Detroit Diesel Field Notes for SPN 91 FMI 2
- Torque accelerator pedal position sensor mounting fasteners per Detroit Diesel 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 Detroit Diesel 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 Detroit Diesel.
- 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 Detroit Diesel 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 Detroit Diesel 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 Detroit Diesel.
- 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.
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 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.
What does SPN 91 FMI 2 mean on Detroit Diesel 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
- Detroit Diesel OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys