Detroit Diesel SPN 91 FMI 4: Accelerator Pedal Position Diagnosis
SPN 91 FMI 4 (FMI 4) — Accelerator Pedal Position 1 — Voltage Below Normal or Shorted to Low Source
Quick Answer
On Detroit Diesel, SPN 91 FMI 4 is a Accelerator Pedal Position fault (Voltage Below Normal or Shorted to Low Source). Prove wiring/live data first; do not parts-cannon the component.
Overview: Detroit Diesel SPN 91 FMI 4
On Detroit Diesel, SPN 91 FMI 4 is a Accelerator Pedal Position fault (Voltage Below Normal or Shorted to Low Source). Prove wiring/live data first; do not parts-cannon the component. This guide covers symptoms, root causes, live-data checks, and misdiagnoses for Detroit Diesel platforms.
Repair Snapshot
Time
1 - 3 Hours
Difficulty
Intermediate
Parts Cost
$50 - $800
Dealer Labor
$350+
Tools Required
- Detroit DDDL8
- Noregon JPRO
- Multimeter
What SPN 91 FMI 4 Means
SPN 91 FMI 4 on Detroit Diesel means Accelerator Pedal Position 1 — Voltage Below Normal or Shorted to Low Source. Confirm 5V ref and signal swing 0.5-4.5V with Detroit DDDL8 / Noregon JPRO before replacing the accelerator pedal position.
Symptoms Drivers Notice
- Check-engine / amber warning lamp illuminated.
- Soft power, unstable idle, or poor response when accelerator pedal position data is irrational.
- Live data for accelerator pedal position fails rationality vs mechanical gauge or partner sensors.
- Codes may return immediately after clearing if root cause remains.
Most Likely Root Causes
Common Misdiagnoses (Don't Waste Parts)
Why it happens:
Do this instead:
Why it happens:
Do this instead:
Why it happens:
Do this instead:
Master Tech Commentary
Mechanic's Notes
Shop tip (Detroit Diesel): For SPN 91 FMI 4, prove 5V ref and signal swing 0.5-4.5V before quoting the Accelerator Pedal Position. Common false path is replacing the sensor when the harness or reference circuit is the failure.
Step-by-Step Diagnostic Procedure
- Step 1: Connect Detroit DDDL8 or Noregon JPRO and record SPN 91 FMI 4 with freeze-frame load/temp.
- Step 2: Inspect harness and connector at the accelerator pedal position for chafe, corrosion, and bent pins.
- Step 3: Verify 5V reference / ground / signal integrity with a multimeter where applicable.
- Step 4: 5V ref and signal swing 0.5-4.5V
- Step 5: Repair the proven fault, clear only after repair, and verify under the same conditions as freeze-frame.
Live Data & Freeze-Frame Tips
Log Accelerator Pedal Position with Detroit DDDL8 under freeze-frame conditions. 5V ref and signal swing 0.5-4.5V. Compare to a mechanical gauge when pressure/temperature related.
When to Limp In vs Tow
Drive carefully to a shop if lamps are warning-only and power remains controllable; tow if power loss is severe.
Related Fault Codes
Frequently Asked Questions
What does Detroit Diesel SPN 91 FMI 4 mean?
It indicates a Accelerator Pedal Position fault: Voltage Below Normal or Shorted to Low Source. Confirm with live data and wiring tests before parts replacement.
Can SPN 91 cause a derate?
It can contribute to limp behavior if the ECM loses trust in critical data, especially when paired with other faults.
What tools diagnose this best?
Detroit DDDL8 and Noregon JPRO, plus a quality multimeter and mechanical gauges for pressure/temperature checks.
Should I clear the code first?
No. Capture freeze-frame and the full stack first. Clear only after the repair and verification.
More Detroit Diesel codes on this system
- Detroit Diesel SPN 2659 FMI 18 — Turbocharger
- Detroit Diesel SPN 91 FMI 2 — Accelerator Pedal Position Sensor
- Detroit Diesel SPN 1569 FMI 31 — Engine Protection Torque Derate
- Detroit Diesel SPN 168 FMI 0 — Battery / ECU Power Supply Voltage
- Detroit Diesel SPN 723 FMI 2 — Camshaft Position Sensor
- Detroit Diesel SPN 1188 FMI 3 — Turbocharger Wastegate Actuator 1 Position
- Detroit Diesel SPN 513 FMI 2 — Actual Engine Percent Torque
- Detroit Diesel SPN 100 FMI 17 — Engine Oil Pressure
Guides that go deeper
- SPN 1569 FMI 31: Engine Protection Shutdown and Safe Recovery Steps [2026 Diagnostic Guide]
SPN 1569 FMI 31 means engine protection shutdown is active—not the root fault. Read stacked codes, verify oil and coolant, then reset only after the trigger is fixed.
- Detroit DDDL Basics: Finding the Root Code Behind a Derate
Use Detroit DDDL the right way: separate root SPN/FMI faults from inducement result codes, read derate stage data, and stop replacing parts on guesses.
- SPN 2791 EGR Valve Actuator Fault: Test Before You Replace Parts
SPN 2791 points at EGR valve/actuator control problems. Confirm command vs actual position, 5V reference, and carbon binding before condemning the cooler.