P1271

P1271 is GM's Accelerator Pedal Position Sensor 1-2 Correlation — the drive-by-wire pedal module has two independent position sensors (APP1 and APP2) wired to the ECM as a redundancy check, and the ECM continuously compares them. The two sensors run on opposite slopes by design: APP1 typically sweeps 0.5V at rest to about 2.5V at full pedal, and APP2 sweeps 1.0V at rest to about 5.0V at full pedal (or similar paired ratios depending on year). When the two signals disagree by more than roughly 10% deviation at the same pedal position for a sustained sample period, the ECM sets P1271 and forces a reduced-power mode (limp-home, ~30% throttle authority) for safety. Cheapest-first ladder: (1) Scan live data and watch both APP1 and APP2 voltages while slowly pressing the pedal — both should sweep smoothly with no dropouts, flat spots, or spikes. A momentary glitch in one signal is the smoking gun. (2) Unplug the pedal connector, inspect for bent pins, green corrosion, or a backed-out terminal, dielectric-grease and reseat. (3) Wiggle-test the harness from the pedal connector back to the ECM looking for chafe-through or a broken wire. (4) Verify the 5V reference and ground at the pedal connector — both APP sensors usually share a 5V supply and a ground; one bad ground throws BOTH signals off and looks like a correlation fault. (5) Replace the pedal assembly ($120-$320 OEM) only after the wiring and grounds check out — most GM pedals are non-serviceable and the sensors are integral to the pedal. Don't replace the pedal before checking the floor mat — a stacked floor mat lifting the pedal off its rest position will throw a correlation code on cold-start KOEO because APP1 and APP2 don't agree on the at-rest baseline.

Vehicle-specific patterns: 2003-2007 Silverado / Sierra Classic with the 4.8/5.3/6.0 Vortec was the first GM platform to widely adopt drive-by-wire pedals, and the pedal modules are known to fail at roughly 100k miles with the contact wipers wearing flat — P1271 paired with P2135 (TPS correlation) on these is almost always the pedal assembly itself. 2005-2010 Cobalt / G5 / HHR with the 2.2/2.4 Ecotec was a heavy P1271 platform — the pedal connector at the firewall bulkhead corrodes from cowl-drain moisture. 2008+ Equinox / Terrain / Traverse with the 3.6 LLT/LFX has a known pedal-harness chafe point where the wiring bends around the brake-pedal bracket. 2007-2010 Silverado HD 2500/3500 with the LBZ/LMM Duramax shares the drive-by-wire pedal architecture with the gas trucks and the same wear-out pattern at high mileage; the diesel's reduced-power mode is particularly noticeable because the trucks are often loaded. Estimated repair: $0 (relocating a stacked floor mat or reseating the connector) to $380 (OEM pedal assembly + harness repair).