Perkins SPN91FMI8 Fault Code: Accelerator Pedal Position #1 : Abnormal Frequency, Pulse Width, or Period
Accelerator Pedal Position #1 : Abnormal Frequency, Pulse Width, or Period · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 91 FMI 8 means the Perkins ECM sees the digital throttle position sensor signal stuck at 0% or 100% duty cycle for more than 2 seconds. The ECM forces throttle position to 0%, may light the warning lamp, and if no working throttle is available the engine drops to and holds at the limp home speed until the fault clears and the keyswitch is cycled.
High severity. The engine keeps running but loses normal throttle control. If both primary and secondary throttle signals are faulted, the machine is stuck at limp home speed, which can strand equipment in traffic, on grades, or mid-task. Treat it as a stop-and-check-soon issue, not an emergency shutdown.
What does Perkins error code SPN91FMI8 mean?
This code applies to the digital throttle position sensor that tells the ECM how far down the accelerator pedal is pressed. That sensor doesn't send a simple voltage: it sends a pulse width modulated (PWM) signal, a constant-frequency square wave where the width of the 'on' pulse (duty cycle) represents pedal position as a percentage from 0 to 100 percent.
SPN 91 FMI 8 fires when that duty cycle signal is pinned at either 0% or 100% for more than 2 seconds, which tells the ECM the signal isn't behaving like a real, moving pedal signal. This can point to a wiring problem, a failed sensor, or a stuck pedal mechanism.
Because most Perkins applications covered by this code use a primary and a secondary throttle position signal, a fault on one side lets the ECM keep running on the other side. It's only when neither throttle signal is usable that the engine is forced down to a fixed limp home speed and held there.
What triggers a Perkins SPN91FMI8 code?
The ECM sets this code when: 1) the throttle position sensor signal frequency reads 0% or 100% for more than 2 seconds, 2) the ECM has been powered for at least 3 seconds, 3) diagnostic codes 91-3 and 91-4 are not currently active, and 4) there are no active 678 or 41 codes. When these conditions are met, the ECM forces the throttle position reading to 0% and, if equipped, turns on the warning lamp. The code logs only if the engine is running, not while cranking.
Common causes of SPN91FMI8
- Digital throttle position sensor signal wiring shorted or open, causing the duty cycle to lock at 0% or 100% instead of varying with pedal movement
- Failed or internally faulty throttle position sensor that can no longer produce a valid PWM signal
- Loss of the sensor's +8 VDC supply from the ECM (fed through the J1 connector) causing the signal to sit at a fixed extreme
- Corroded, loose, or damaged connector terminals at the throttle pedal assembly or ECM connector interrupting the signal
- Mechanical binding or a stuck accelerator pedal assembly holding the sensor output at one extreme
How to troubleshoot Perkins SPN91FMI8: first checks
- Check the warning lamp and use an electronic service tool to confirm SPN 91 FMI 8 is active or logged, and check whether 91-3, 91-4, 678, or 41 codes are also present since those affect how this code sets
- Inspect the throttle pedal wiring harness and connectors for corrosion, loose pins, chafing, or pinched wires between the pedal assembly and the ECM
- Verify the pedal moves freely through its full travel with no mechanical binding or debris restricting motion
- Confirm the digital throttle position sensor is receiving its +8 VDC supply from the ECM at the J1 connector feed to the sensor
- Check whether the fault is on the primary or secondary throttle circuit, since the engine can run on the good side while the other is repaired
- Cycle the keyswitch after any repair, since the ECM will keep ignoring inputs from the repaired throttle until the keyswitch is cycled
How the code clears
No separate clearing step is listed beyond repairing the fault. Once the wiring, sensor, or mechanical issue is corrected, the ECM will continue to ignore inputs from the repaired throttle circuit until the keyswitch is cycled. Cycling the key allows the ECM to re-read the repaired signal and resume normal throttle control.
Frequently asked questions
What does SPN 91 FMI 8 mean on a Perkins engine?
It means the ECM sees the digital accelerator pedal position sensor signal stuck at either 0% or 100% duty cycle for more than 2 seconds, instead of a normal signal that varies with pedal position. The ECM treats this as an abnormal frequency, pulse width, or period fault.
Will my Perkins engine still run with this code active?
Yes, usually. If the fault is only on one throttle signal (primary or secondary), the ECM switches to the working signal and the engine runs normally until the fault is repaired. Only if no functional throttle signal is available does the engine drop to and hold at the limp home speed.
Why is my engine stuck at one speed and won't respond to the pedal?
If neither the primary nor secondary throttle signal is working, the ECM defaults to a fixed limp home speed. It will decelerate down to that speed if running faster, or hold its current speed if already running slower, and stay there as long as the code is active.
Do I need to cycle the key after fixing the wiring or sensor?
Yes. Even after the throttle circuit is repaired, the ECM will continue ignoring inputs from that repaired throttle until the keyswitch is cycled. Turn the key off and back on to let the ECM resume using the fixed signal.
Does PTO mode affect this code?
If the application uses the ECM's dedicated PTO functions, the digital throttle position sensor is ignored while the engine is in PTO mode. PTO mode status can be checked with an electronic service tool by confirming whether the PTO ON/OFF switch is ON.
What's the difference between this code and 91-3 or 91-4?
SPN 91 FMI 8 specifically requires that 91-3 and 91-4 are not active at the same time, since those cover different signal fault conditions on the same sensor. If 91-3 or 91-4 is active, the ECM handles that as a separate diagnostic condition.
Does the warning lamp always come on for this code?
The lamp comes on if the machine is equipped with one. The diagnostic code itself logs only while the engine is running, and it will not log if the fault occurs while the engine is cranking.