P2123

P2123 is the high-voltage counterpart to P2122 on APP sensor D, meaning the signal voltage has climbed above roughly 4.7V when the pedal is at rest or above the expected sweep range during operation. The vehicle drops into limp mode with reduced throttle authority. Cheapest-first diagnosis: key on engine off, scan APP-D voltage and watch it at rest (expect 0.5 to 0.9V) and through a smooth sweep to wide-open throttle (expect 3.8 to 4.5V max). A reading pegged at 5V at rest indicates either the signal wire shorted to the 5V reference, a sensor with an internal short-to-5V, or an open ground inside the pedal assembly causing the signal to float up. Back-probe the APP-D ground wire at the pedal connector with the key on, it should read under 0.1V; anything higher confirms a bad ground path and the fix is harness-side rather than sensor-side. Wiggle the harness from pedal to firewall while watching live data for the voltage to drop, that pinpoints the chafe location. The expensive misdiagnosis is condemning the PCM when a single corroded ground pin or a wire chafed against a metal bracket is the cause.

Vehicle-specific patterns: 1999 to 2007 GM full-size trucks set P2123 from the same APP harness corrosion pattern as P2122, water-soaked carpet under the driver foot well corrodes the pedal connector and floats the signal voltage high, GM TSB 04-06-04-049. 2004 to 2008 Ford F-150 sets P2123 from internal APP sensor track wear at the rest position. 2003 to 2010 Dodge Ram sets P2123 from a failed APP module ground inside the pedal assembly. 2005 to 2012 Nissan Frontier and Pathfinder set P2123 from APP sensor connector corrosion at the pedal. Estimated repair: $35 to $420.