SPN 91 FMI 1 indicates the accelerator pedal position signal is below the calibrated minimum threshold — a low-signal or short class fault, or an implausibly low pedal value that the ECM cannot rationalize. Signal short to ground, a failed pedal sensor track, open 5V reference, connector corrosion, and harness pinching at the firewall are the primary causes. Limp-mode events from one failed APP channel and water intrusion in floor-mounted connectors are the most documented patterns. Replacing the pedal assembly before proving the circuit is the most common misdiagnosis.

Inspect pedal connector cavities for moisture and corrosion — floor-mounted connectors are directly exposed to water intrusion. Check firewall transition points for pinch damage. Verify ground eyelet integrity at the chassis ground point nearest the pedal. Inspect harness routing for stretched wires caused by repeated full-travel pedal cycling.

Dual-path APP systems maintain two independent signal tracks that must remain correlated within OEM tolerance. Confirm 5V reference and ground at the pedal connector before condemning the sensor. A channel pinned near 0V with the pedal at rest supports a short to ground or a failed sensor track. Graph both channels simultaneously through a full pedal sweep — correlation failure between channels is the key diagnostic indicator.