What is SPN 91 FMI 8 (Caterpillar) & How to Solve It?

What is SPN 91 FMI 8 (CAT):

SPN 91 relates to the Accelerator Pedal Position Sensor (APPS), and FMI 8 means the sensor is abnormal but still within the working range.

On Caterpillar machines, this code typically signals that the throttle pedal is sending inconsistent or mismatched signals — causing unresponsive throttle or erratic acceleration behavior.

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Why This Happens

• Sensor beginning to drift or wear out.
• Electrical interference in the pedal signal wires.
• Dual pedal sensors reporting values that don’t match.
• Connector pins loose or corroded.
• ECM misinterpreting signals due to software mismatch.

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How to Fix It

1. Inspect the pedal sensor connector and wiring for damage.
2. Use a scan tool to monitor both sensor signals — compare expected vs actual values.
3. Replace the pedal assembly if one channel is drifting.
4. Check for calibration issues in the ECM.
5. Clear the code and test throttle responsiveness after repair.

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How to Avoid It Before It Happens

1. Include pedal sensor testing during PM inspections.
2. Replace pedals with aging sensors before failure occurs.
3. Protect wiring harnesses from water intrusion under the cab.
4. Educate operators to report unresponsive or jumpy acceleration.