P2120

P2120 means the ECM has detected a circuit fault on the 'D' channel of the throttle/pedal position sensor — on most platforms, 'D' is one of the two redundant accelerator pedal position (APP) sensors mounted in the pedal assembly itself. The pedal has two independent potentiometers (APP-D and APP-E) wired through separate ECM pins, and they're calibrated to sweep opposite directions as the pedal moves so the ECM can cross-check them. P2120 specifically means the 'D' channel is open, shorted, or reading a value that's electrically impossible (below ~0.2V or above ~4.8V on most platforms). Cheapest-first: unplug the pedal-assembly connector, inspect for backed-out pins, corrosion, or melted plastic, and reseat with dielectric grease. Then backprobe APP-D at the pedal connector with the key on — you should see roughly 0.5V at rest and roughly 4.5V at full pedal travel, with a smooth linear sweep in between (a scope is better than a DMM here — a flat-spotted wiper element looks fine on a meter but shows a dropout on a scope). If the wire from pedal to ECM is good but the sensor doesn't sweep linearly, the pedal assembly is the fault — and on most platforms the pedal sensor isn't serviceable separately from the pedal arm.

Vehicle-specific patterns: 2007-2014 GM full-size trucks (Silverado / Sierra / Tahoe / Suburban) with the 5.3L / 6.0L Vortec is the dominant P2120 platform — the APP harness runs under the driver's-side carpet and gets stepped on, water from wet boots wicks into the connector, and the D-channel pin corrodes first because of its position in the connector body. The fix is repinning or replacing the pedal-side pigtail and resealing the connector. 2006-2010 Dodge Ram 1500 / 2500 throws P2120 from the same harness-corrosion pattern. 2008-2013 Ford F-150 5.4L 3V sees P2120 from internal pedal-sensor wear where the wiper flat-spots at the rest position from years of foot pressure. 2010-2015 Chevy Camaro / Cadillac CTS gets P2120 from a chafed harness where it routes through the firewall grommet. Critical: a single bad pedal sensor will throw P2120 alongside a forced-idle code (P2104, P2110, or P2135), so confirm the pedal is the fault before throwing parts — the throttle body itself is innocent.