SPN 91 FMI 3 — Accelerator Pedal Position Circuit Voltage Above Normal
SPN 91 FMI 3 is a high-voltage class fault on the accelerator pedal position circuit. This is a safety-critical diagnosis — an open signal circuit pegged high can cause limp-mode activation. Dual-channel correlation logic prevents unnecessary ECM replacement.
Fault Overview
FMI 3 on the APP circuit indicates the signal is above the valid maximum voltage range. On 5V ratiometric dual-path APP systems, an open signal circuit causes the signal to float toward the reference voltage, producing a high-voltage reading. A failed pedal sensor, poor connector tension, and broken 5V or ground reference causing implausible outputs are the primary causes. Pedal assembly wear and open-circuit channels under vibration are documented patterns. Dual APP paths should remain correlated — a path pegged high while the other reads normally indicates a channel-specific fault rather than an ECM issue.
Engine & Platform Coverage
Symptoms
- •Accelerator pedal high voltage fault active
- •Throttle sensor open indication
- •Limp mode from pedal code
- •Power limit from APP fault
- •Intermittent loss of throttle response
Possible Causes
- 1.Open signal circuit — signal floating toward reference voltage
- 2.Failed pedal sensor — internal open on APP channel
- 3.Poor connector tension — pin pullout or corrosion
- 4.Broken 5V or ground reference — causing implausible outputs
- 5.Pedal assembly wear — open-circuit channel under vibration
- 6.Stretched wires from pedal travel — firewall routing damage
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Single Action Gate — Graph APP Channels: Graph both APP channels simultaneously. Dual APP paths should remain mathematically correlated throughout the pedal travel range. A channel pegged high while the other reads normally confirms a channel-specific fault. Both channels pegged high suggests a reference or ground issue.
- 2Binary Decision Gate — Reference and Ground Check: Verify 5V reference and ground at the pedal connector. An absent 5V reference causes the signal to behave abnormally. A missing ground causes both channels to read implausibly. Confirm both are present before evaluating the pedal assembly.
- 3Override Manager — Harness Wiggle Test: Wiggle the harness at the pedal connector and along the firewall routing while monitoring APP channel PIDs. Stretched wires from pedal travel and firewall routing damage are documented causes of intermittent high-voltage faults.
- 4Safety Interceptor — Connector Pin Tension Check: Inspect APP connector pin tension and condition. Poor pin tension causes intermittent open-circuit conditions that produce high-voltage readings under vibration. Inspect for pin pullout and corrosion.
- 5Resume Point — Pedal Assembly Evaluation: If circuit integrity is confirmed and the fault persists, evaluate the pedal assembly. Pedal assembly wear producing open-circuit channels under vibration is a documented pattern on high-mileage vehicles.
- 6Verification Loop — Post-Repair Channel Confirmation: After repair, verify both APP channels are correlated and within OEM specification throughout the pedal travel range. Confirm limp mode does not activate during a test drive.
- 7Repair Ledger Output — Pedal or Harness Decision: Replace pedal assembly after circuit integrity is confirmed and pedal failure is proven. Repair harness if open wire or stretched conductor is found. Never replace ECM for APP high-voltage without proven circuit and pedal integrity.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair open wire or stretched conductor in APP harness.
- 2Repair connector pin tension or corrosion.
- 3Replace pedal assembly after circuit proof confirms pedal failure.
- 4Verify both APP channels are correlated after repair.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
SPN 91 FMI 3 is a circuit and pedal diagnosis with safety impact. The AI Diesel Tech FSM enforces dual-channel correlation logic to prevent unnecessary ECM replacement on APP high-voltage faults.