SPN 91 FMI 2 — Accelerator Pedal Position Data Erratic
Erratic APP data is dual-channel integrity and supply stability diagnosis. ECM replacement attempts for intermittent APP correlation faults — when the pedal tracks or harness were the actual cause — is the most documented misdiagnosis.
Fault Overview
SPN 91 FMI 2 indicates the accelerator pedal position signal is unstable, implausible, or dual-channel correlation is unstable. Failing pedal tracks, intermittent harness, poor 5V or ground, connector fretting, and water intrusion at floor connectors are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Failing pedal track with intermittent dropout under vibration
- 2.Intermittent harness at firewall transition or floor connector
- 3.Poor 5V supply stability at pedal connector
- 4.Poor ground eyelet integrity at pedal or chassis
- 5.Water intrusion at floor-area connectors
Wiring & Circuit Notes
Graph both APP channels through full pedal sweep and at steady pedal positions. Dual tracks must remain correlated. Dropouts under vibration are harness or pedal. Ground eyelet integrity at the pedal and chassis is a common overlooked cause.
DVOM Specs & Sensor Tests
Graph both APP channels simultaneously through full sweep and at steady positions. Dual tracks should remain correlated within OEM specification. Check 5V supply and ground stability at the pedal connector. Wiggle test through pedal travel and harness. Disconnect isolation if shorts are suspected. Replace pedal assembly only after circuit is proven.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Graph both APP channels simultaneously through full pedal sweep
- 2Check 5V supply and ground stability at pedal connector
- 3Wiggle test through pedal travel and harness at firewall and floor
- 4Duplicate fault under hot and vibration conditions
- 5Disconnect isolation if short is suspected on one channel
- 6Replace pedal assembly after circuit is proven
- 7Verify correlation stability under road load before release
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
5 steps locked- 1Graph both APP channels simultaneously through full pedal sweep
- 2Check 5V supply and ground stability at pedal connector
- 3Wiggle test through pedal travel and harness at firewall and floor
- 4Duplicate fault under hot and vibration conditions
- 5Disconnect isolation if short is suspected on one channel
- 6Replace pedal assembly after circuit is proven
- 7Verify correlation stability under road load before release
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.
Related Resources
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
SPN 91 FMI 2 is dual-channel integrity and supply stability diagnosis. Both channels and supply must be proven before pedal or ECM replacement.