SPN 91Accelerator pedal position (APP) signal fault

Is the J1939 accelerator pedal position parameter on Class 7–8 diesels and many vocational trucks. When it faults, the ECM no longer trusts the driver’s foot request—idle validation fails, dual APP channels disagree, or the signal is open/shorted—so the truck often derates, sticks at idle, or enters a limp “pedal to idle” strategy. Drivers feel no throttle response, a sudden power cut on the highway, or an APP / check-engine / stop-engine lamp depending on OEM. Read the FMI with the SPN before you wrench: data-erratic pushes toward dual-track disagreement and harness flex; voltage-high/low pushes toward open or shorted circuits.

Shop frequency: failed APP sensor assembly (worn dual potentiometers or Hall tracks), then harness chafe at the pedal or firewall grommet, then connector fretting after cab work. Next: water intrusion in the pedal connector, wrong pedal installed after a cab swap, or rare ECM input faults. Do not condemn the ECM because throttle feels dead—prove both APP channels agree at idle, mid, and WOT percentages against the OEM chart.

Diagnosis: graph APP channel 1 and 2 (or percent and redundant percent) while sweeping the pedal slowly; they should track in a fixed offset relationship. Key-off ohm and voltage-test supply, grounds, and signals at the pedal connector. Wiggle the harness at the pedal pivot and through the firewall while watching for dropouts. Replace the APP assembly when channels disagree or open with a healthy harness. Dig elsewhere when a new pedal still fails with a chafed loom, when both channels die together (shared 5 V/ground), or when clutch/idle validation switches are the real enable blockers. System context: SPN 91 is the torque request into the ECM—without it, fueling and transmission shift requests collapse to a safe idle. Fix the pedal path before chasing turbo or aftertreatment ghosts.