What Is Fault Code SPN 91 FMI 8? (Accelerator Pedal Position)

The lists do not always cover every code or every issue you need at the time. The official SPN fault code list starts at 100001 and FMI 0. That means it does not go below 100 when you get to the 3-digit code section.

What the code SPN 91 FMI 8 means, in spite of the exclusion of the best fault code list, is that the throttle position signal is abnormal. This is not a good code as your engine may be limited to just a low idle and no higher. To figure out the problem you have to test the throttle position sensor circuit.

To learn more about this code and what it means, just continue to read our article. It has the information you want to know about so you can get to the heart of the problem quickly and get back on the road even faster.

What is The Code SPN 91 FMI 8


Not being on the official list is okay. There are plenty of other sources available online that provide detailed information about this code. What this code means is- Accelerator Pedal or Lever Position Sensor 1 Circuit Frequency - Abnormal Frequency or Pulse Width or Period.

As usual, you have to clear other fault codes before working on this one. Especially these two codes if they have shown to be active in the last 25 hours: Sensor supply 4: 2185, 2186.

There are 9 pages of repairs, validations, etc., for this code. Part of the repair will be to check for voltage issues, open or shorted circuits, and so on. In the end, you may find that the gas pedal needs to be changed as it is malfunctioning.

To read the long list of instructions, just click here. It is possible you may have to replace the OEM wire harness as well.

How do I Know If My Accelerator Pedal Position Sensor is Bad?

In the last century auto and truck makers only used a cable for the throttle position. When that went bad it was an easy fix. In today’s vehicles, everything is done electronically including the throttle position.

There are 6 signs that the sensor for this component has failed on you:

1. You feel a hesitation in your vehicle when you press down on the accelerator

2. The idle sounds rough and if the fuel-air mixture is okay, then the throttle pedal sensor is going bad.

3. You cannot accelerate beyond a certain speed. That means that the sensor pedal sensor cannot send any accurate position signals.

4. There may be issues with your shifting. If the sensor is failing, it can’t send proper signals to the automatic transmission computer.

5. You are losing fuel efficiency. The sensor is sending inaccurate signals to the ECM which then sends too little or too much fuel to the engine.

6. Your check engine light will be illuminated and your code reader will provide you with the SPN 91 FMI 8 code.

SPN 91 FMI 8 Caterpillar

SPN-91-FMI 8-Caterpillar

The equivalent code for CAT is MID 36 CID 91. As you know, MID stands for modular Identifier and 36 means Engine Control #1. That is the module where you have to look first to help solve this problem.

Then CID stands for Component Identifier and 91 points to the exact location of this problem. It is the throttle position sensor. Since this code is basically the same as the SPN code, then the repair should be the same as well.

Be prepared to have your engine derated to low idle when you see this code flashed onto your dash’s screen. You should have to also perform the throttle position sensor circuit test.

This sensor replaces the linkage that used to be attached to throttles in all vehicles. When operating correctly it uses a signal that is Pulse Width Modulated.

This produces a duty cycle that varies as you press or let up on the throttle. Idle is a low-duty cycle and acceleration is a high-duty cycle. Then the ECM processes the signal to set the speed of the engine.

This cannot be done if the sensor is going bad on you. What activates the SPN 91 FMI 8 code is when the sensor does not send out a signal to the ECM between the 150 to 1050 Hz levels.

To get the many steps to fix this problem on a CAT, just click here and scroll down to Test Step 1. It is just after the wiring diagram.

SPN 91 FMI 8 Freightliner

This may be called a low voltage check when you see this code. To start troubleshooting this problem you will have to disconnect the throttle and turn the key to the ON position. Make sure not to start the engine.

There are 10 steps to this repair and you may or may not have to go through all 10. We have given the first two already and the 3rd is:

Measure the voltage between pins B and C of the AP harness connector.

[a] If the voltage is between 4.5 and 5.5 volts, go to step 5.

If the voltage is less than 4.5, go to step 4.”

We won’t provide all the steps here but 4 & 5 will be quoted and they are:

4. Measure the voltage between pin C of the AP harness connector and the ground.

[a] If the voltage is between 4.5 and 5.5 volts, repair the open circuit between pin B of the

AP harness connector and pin 4 of the CPC #1 connector. Refer to section

If the voltage is less than 4.5, repair the open circuit between pin C of the AP harness connector and pin 8 of the CPC #1 connector. Refer to section

5. Turn OFF the ignition.

If this does not solve your problem, then click here to get to the rest of the steps.

SPN 91 FMI 4


This code means- the circuit failed low. The information comes from the Detroit Diesel Engine troubleshooting manual section 22.3. However, there is some not so good news.

There is no different repair process than the previous fault code for the Freightliner section immediately above. You can go to the link above and read the rest of those steps. That would be the best thing to do.

Mostly what you are doing is measuring the ohms between different pins. The exact pin number is in those instructions. The key would be to be above 5 ohms at every step of the way.

If you are under 5 ohms, then you will have to replace or repair some wires between the pins you are measuring.

SPN 91 FMI 2

There is some good news here. This code actually appears on the Cummins fault code list and its equivalent number is 1242. The bad news is that when you see this code, you should be seeing a red light illuminated and not the amber one.

What this code means is Accelerator Pedal Position 1 Accelerator Pedal or Lever Position Sensor 1 - Data erratic, intermittent, or incorrect. We said bad news because you will be seeing the red light and that is because your engine will only be able to idle until you clear the code.

What causes this specific fault code is that the throttle position sensor was damaged. To get the repair instructions you have to click here. This link takes you to a website that does not allow copying and pasting.

SPN 91 FMI 3


This fault code also appears on the complete Cummins fault code list and its equivalent number is 1358. When set, you should only see an amber light and the code means:

Accelerator Pedal Position 1 Accelerator Pedal or Lever Position Sensor 1 Circuit - Voltage above normal, or shorted to high source “ There is more good news as this code appears twice on the Cummins fault code list.

The other equivalent number is 131. When you see this number, there should be a red light illuminated as well. This code level will put you in limp mode. There are two different repair guidelines to follow.

One is at the Detroit Diesel troubleshooting manual Series 60 and it is at this link. The other is at this link, the website that does not allow for copying and pasting.

Some Final Words

There is always a way to find fault codes when they are not included on the main lists. Sometimes you just have to search for someone who is asking a mechanic what the code means as they also can’t find information on it.

Once you get the information, then the repair is only a matter of time. Just so you know, SPN 91 FMI 4 is also on the complete Cummins code list twice. Why, we do not know but at least they are there.

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