SPN 91 FMI 3 Kenworth X15 — Accelerator Pedal Positio (2026)
How We Tested and Verified
SPN 91 FMI 3 on Kenworth X15 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 3 on Kenworth X15 means accelerator pedal position 1 - voltage above normal, circuit shorted high. 60–80% of field cases trace to pinched or corroded 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 3 on Kenworth X15 tells the ECM that accelerator pedal position sensor circuit is outside calibrated limits. On J1939, FMI 3 specifies the failure mode: Accelerator Pedal Position 1 - Voltage Above Normal, Circuit Shorted High. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.
Common Symptoms
- Engine limited to idle or a low fixed power state (100% of cases)
- Amber or red warning lamp depending on severity (80% of cases)
- No throttle response from the accelerator pedal (60% of cases)
- Fault may be intermittent if caused by a vibration-related short (40% of cases)
Likely Causes (Ranked by Probability)
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Pinched or corroded pedal connector (45% of cases) — The low-mounted connector is exposed to moisture and floor debris.
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Chafed harness near the floor bracket (25% of cases) — Repeated pedal movement flexes the harness at a consistent point.
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Failed sensor internally shorted (15% of cases) — Less common than wiring but occurs with age.
Step-by-Step Diagnostic Guide
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Inspect the pedal assembly connector for a pinched wire or corrosion at the pins. — Tool: scan tool or multimeter. Document readings on the work order.
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Backprobe the signal wire with key on, engine off - a voltage pegged near supply voltage confirms the short. — Tool: scan tool or multimeter. Document readings on the work order.
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Check harness routing near the floor bracket and firewall grommet for chafing. — Tool: scan tool or multimeter. Document readings on the work order.
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Compare both redundant pedal position channels on a scan tool. — Tool: scan tool or multimeter. Document readings on the work order.
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Replace the pedal assembly only after wiring has been confirmed clean. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Accelerator Pedal Position Sensor Circuit wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Accelerator Pedal Position Sensor Circuit sensor/part | $150 - $400 | $$150 - $400 + labor | $350+ |
| Full module replacement | N/A | $600–$1,500 | $350+ |
Platform-Specific Notes
On Kenworth X15, 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 3 sets when accelerator pedal position sensor circuit 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 Kenworth captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Accelerator Pedal Position Sensor Circuit 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 pedal assembly immediately — It’s a fast bolt-in fix.. Instead: Backprobe the wiring first - many cases are connector or harness issues.
Suspecting the ECM — A sudden idle-only state feels like a control module problem.. Instead: Cross-check both redundant pedal channels.
Shop Notes from the Field
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.
Safety Warning
- Do not drive if throttle response is unpredictable.
- Verify normal pedal return before test-driving after repair.
Prevention Tips
- Protect the pedal connector from underfloor moisture.
- Inspect the harness during PM services.
- Address early throttle hesitation before it becomes a hard fault.
Kenworth Field Notes for SPN 91 FMI 3
- Document ambient temperature and key-cycle count at fault set on Kenworth X15 — cold-soak faults on accelerator pedal position sensor circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 91 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If accelerator pedal position sensor circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Kenworth.
- Capture a 20-minute data log of accelerator pedal position sensor circuit 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 3 on X15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque accelerator pedal position sensor circuit mounting fasteners per Kenworth 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 Kenworth X15 — cold-soak faults on accelerator pedal position sensor circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 91 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If accelerator pedal position sensor circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Kenworth.
- Capture a 20-minute data log of accelerator pedal position sensor circuit 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 3 on X15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque accelerator pedal position sensor circuit mounting fasteners per Kenworth OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
Related Codes
- SPN 91 FMI 4 — often stacks with SPN 91 FMI 3
FAQ
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.
What does SPN 91 FMI 3 mean on Kenworth X15?
SPN 91 FMI 3 means accelerator pedal position 1 - voltage above normal, circuit shorted high. The ECM detected accelerator pedal position sensor circuit outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 91 FMI 3 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 3?
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 3?
Pinched or corroded pedal connector — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 91 parameter definition
- Kenworth OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys