SPN 91 FMI 10 – Throttle pedal rationality check failed Fault Code (Symptoms, Causes & Fix)

Mar 15, 2026

The SPN 91 FMI 10 fault code indicates a throttle pedal rationality check failure in a diesel engine. This diagnostic trouble code suggests that the Engine Control Module (ECM) has detected an inconsistency in the throttle pedal position sensor's signal, which is crucial for accurate engine throttle control.

What SPN 91 FMI 10 Means

The SPN 91 FMI 10 fault code is triggered when the ECM identifies a discrepancy between the expected and actual signals from the throttle pedal position sensor. This sensor is responsible for communicating the driver's throttle input to the ECM, which then adjusts engine parameters accordingly. A failure in this rationality check means the ECM cannot trust the signal, potentially affecting engine performance.

Common Causes of This Fault Code

Failed throttle pedal position sensor
Wiring harness damage
Connector corrosion or looseness
ECM signal processing issues
Voltage reference faults

Symptoms Drivers May Notice

When the SPN 91 FMI 10 fault code is active, drivers may experience symptoms such as erratic throttle response, reduced engine power, or unexpected engine derate. In some cases, the engine may enter a protection mode to prevent further damage.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 91 FMI 10 fault by performing a thorough visual inspection of the throttle pedal and its connectors for any signs of physical damage or corrosion. Next, they use a multimeter to check for continuity and proper voltage at the throttle pedal position sensor, ensuring the sensor receives the correct power supply and ground.

A wiggle test on the wiring harness is conducted to identify any intermittent faults. Technicians also utilize diagnostic software to monitor live data from the throttle pedal position sensor, comparing it to expected values to verify the sensor's accuracy. Additionally, they check for any related fault codes that might indicate broader issues with the ECM.

Common Misdiagnosis

Technicians often misdiagnose this fault by replacing the throttle pedal position sensor without verifying the integrity of the wiring. Overlooking intermittent wiring issues due to improper testing is another common mistake. It's also crucial not to assume a mechanical issue with the throttle linkage when the problem is electrical.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 91 FMI 10 fault code active, it's not advisable. The fault can lead to engine derate or protection mode, significantly affecting vehicle performance and potentially causing unsafe driving conditions. Addressing the issue promptly is recommended to avoid further complications.

Mechanic's Diagnostic Tip

When diagnosing the SPN 91 FMI 10 fault, always verify the 5-volt reference circuit to the throttle pedal position sensor. A compromised reference voltage can lead to incorrect sensor readings, causing the ECM to misinterpret throttle input.

Frequently Asked Questions

Q: What tools are essential for diagnosing SPN 91 FMI 10?
A: A multimeter and diagnostic software are crucial for testing voltage, continuity, and monitoring live data from the throttle pedal position sensor.

Q: Can a software update resolve SPN 91 FMI 10?
A: While a software update can sometimes address ECM-related issues, it's essential to first verify the sensor and wiring integrity before considering software solutions.

Q: Is it common for the throttle pedal position sensor to fail?
A: While not the most common failure, throttle pedal position sensors can fail due to wear or electrical issues, making it important to test thoroughly before replacement.