P2122

P2122 is the low-voltage fault on Accelerator Pedal Position (APP) sensor D, the first of the redundant pair inside the drive-by-wire pedal assembly. The PCM expects APP-D to sit at roughly 0.5 to 0.9V at rest and sweep to 3.8 to 4.5V at full throttle, and P2122 sets when the voltage drops below roughly 0.3V with the key on. The vehicle typically goes into reduced-power limp mode with a hard RPM cap. Cheapest-first diagnosis: key on engine off, scan APP-D voltage at rest, it should read 0.5 to 0.9V; press the pedal smoothly to the floor and watch for a clean linear sweep to 3.8 to 4.5V. A reading stuck at 0V or jumping to 0 mid-sweep indicates either a sensor failure or, far more commonly, a chafed or corroded harness. Back-probe the APP-D 5V reference, signal, and ground wires at the pedal connector, the 5V ref should hold steady at 4.9 to 5.1V regardless of pedal position. If the signal wire reads 0V while the 5V ref is good, the fault is between the pedal and the PCM. Inspect the harness where it enters the firewall and where it lies under the carpet on GM trucks. The expensive misdiagnosis is replacing the pedal assembly (typically $200 to $400) when the actual fault is a $5 length of harness repair under the carpet.

Vehicle-specific patterns: 1999 to 2007 GM full-size trucks (Silverado, Sierra, Tahoe, Suburban, Yukon) set P2122 from APP pedal harness corrosion where the harness runs under the driver-side carpet, water from boot snow soaks the foam padding and corrodes the connector pins, GM TSB 04-06-04-049 covers harness repair. 2003 to 2009 Dodge Ram with electronic pedal sets P2122 from a failed APP sensor, internal carbon track wear. 2005 to 2010 Ford F-150 sets P2122 from a chafed APP harness at the firewall pass-through grommet. 2004 to 2008 Toyota Tundra 4.7L sets P2122 from internal APP sensor failure. Estimated repair: $35 to $420.