Mack SPN 91 FMI 9: Accelerator Pedal Position Diagnosis
SPN 91 FMI 9 (FMI 9) — Accelerator Pedal Position 1 — Abnormal Update Rate
Quick Answer
On Mack, SPN 91 FMI 9 is a Accelerator Pedal Position fault (Abnormal Update Rate). Prove wiring/live data first; do not parts-cannon the component.
Overview: Mack SPN 91 FMI 9
On Mack, SPN 91 FMI 9 is a Accelerator Pedal Position fault (Abnormal Update Rate). Prove wiring/live data first; do not parts-cannon the component. This guide covers symptoms, root causes, live-data checks, and misdiagnoses for Mack platforms.
Repair Snapshot
Time
1 - 3 Hours
Difficulty
Intermediate
Parts Cost
$50 - $800
Dealer Labor
$350+
Tools Required
- Premium Tech Tool
- Noregon JPRO
- Multimeter
What SPN 91 FMI 9 Means
SPN 91 FMI 9 on Mack means Accelerator Pedal Position 1 — Abnormal Update Rate. Confirm 5V ref and signal swing 0.5-4.5V with Premium Tech Tool / 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:
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Why it happens:
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Master Tech Commentary
Mechanic's Notes
Shop tip (Mack): For SPN 91 FMI 9, 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 Premium Tech Tool or Noregon JPRO and record SPN 91 FMI 9 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 Premium Tech Tool 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 Mack SPN 91 FMI 9 mean?
It indicates a Accelerator Pedal Position fault: Abnormal Update Rate. 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?
Premium Tech Tool 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 Mack codes on this system
- Mack SPN 190 FMI 0 — Engine Speed Sensor Circuit
- Mack SPN 1569 FMI 31 — Engine Protection Torque Derate
- Mack SPN 100 FMI 18 — Engine Oil Pressure Sensor / Oil Pressure Circuit
- Mack SPN 168 FMI 1 — Battery / Charging System Circuit
- Mack SPN 98 FMI 17 — Engine Oil Level Sensor
- Mack SPN 646 FMI 5 — Turbocharger 1 Wastegate Drive
- Mack SPN 190 FMI 8 — Engine Speed Sensor (Crankshaft)
- Mack SPN 636 FMI 2 — Crankshaft Position Sensor
Guides that go deeper
- 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.
- 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.