What are the symptoms of SPN 91 FMI 3?

Accelerator pedal position reading stuck at maximum or above normal range. Engine may surge or run at high RPM without pedal input. Engine derate or idle-only mode active - ECM defaults to safe state. No throttle response or erratic throttle response. Possible companion faults: SPN 91/4 (APP voltage low), SPN 91/2 (APP erratic). Cruise control may be disabled

What causes SPN 91 FMI 3?

APP sensor signal wire shorted to voltage - 5V reference or 12V supply. APP sensor internal failure - signal circuit shorted high internally. APP sensor connector moisture intrusion - water bridging signal and reference pins. Damaged wiring harness - chafed signal wire contacting power source near pedal assembly. APP sensor 5V reference wire shorted to signal wire - both pins at 5V

How do I diagnose SPN 91 FMI 3?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 91 FMI 3 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 91 FMI 3Fuel SystemHigh

Accelerator Pedal Position 1 - Voltage Above Normal or Shorted High

Run Full Diagnosis View Related Faults See OEM System Guide

Scan Tool — in guided session

DVOM Testing — in guided session

Connector Pinout — in guided session

DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.

Overview

Fault Code

SPN 91 / FMI 3

Common Symptoms

Accelerator pedal position reading stuck at maximum or above normal range
Engine may surge or run at high RPM without pedal input
Engine derate or idle-only mode active - ECM defaults to safe state
No throttle response or erratic throttle response
Possible companion faults: SPN 91/4 (APP voltage low), SPN 91/2 (APP erratic)
Cruise control may be disabled

Root Causes

APP sensor signal wire shorted to voltage - 5V reference or 12V supply
APP sensor internal failure - signal circuit shorted high internally
APP sensor connector moisture intrusion - water bridging signal and reference pins
Damaged wiring harness - chafed signal wire contacting power source near pedal assembly
APP sensor 5V reference wire shorted to signal wire - both pins at 5V

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

Start Guided Diagnosis See How It Works

Derate Behavior

Power Derate

Yes

Mileage / Regen Notes

APP sensor failure increases with mileage and pedal wear. Connector moisture intrusion increases in vehicles with cab floor leaks.

System Interactions

SPN 91 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.

Fuel rail pressure — injector timing and quantity affected

Engine power output — derate may be triggered to protect injectors

Aftertreatment regen — low fuel pressure can prevent regen completion

TSB Reference: Cummins TSB 4021756 - Accelerator Pedal Position Sensor Diagnosis; Detroit TSB 17-008 - APP Sensor Voltage High Diagnosis

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing injectors without verifying fuel supply pressure — low supply pressure causes injector faults.

Ignoring fuel filter restriction — a clogged filter causes low rail pressure that mimics injector failure.

OEM Notes

Cummins

X15: APP sensor is integrated into pedal assembly - replace entire assembly if sensor fails. INSITE APP sensor test required. Verify both APP1 and APP2 readings match.

Detroit

DD15: APP sensor connector is located under the dash - inspect for moisture from cab floor leaks. DDDL APP sensor test required.

PACCAR

MX-13: APP sensor wiring harness routing under the cab floor is a common chafe point. Inspect harness routing carefully.

Volvo

MP8: APP sensor failure on MP8 often causes engine to default to idle-only mode. PTT APP sensor test required. Verify both APP1 and APP2 are within 5% of each other.