P2138

P2138 sets when the PCM sees the two redundant signals inside the accelerator pedal position (APP) sensor disagree outside the calibrated correlation window, which is typically more than about 10 percent variance for longer than a half second. By design the ECU goes into reduced-power mode (often called limp home or forced idle) the instant this fault is active, because the throttle-by-wire system has no mechanical cable to fall back on. Diagnose cheapest-first: scan live data and watch APP1 and APP2 voltage sweep from idle to wide-open and look for a glitch, dropout, or non-linear step in either trace, then unplug the pedal connector and inspect for green corrosion, bent pins, or backed-out terminals (this alone resolves a huge percentage of cases). If both signals sweep cleanly at the pedal but the PCM still disagrees, back-probe the two signal wires at the PCM connector while flexing the harness to find a broken conductor. Caveat: replacing the pedal assembly without first inspecting the connector and harness is the single most common wasted parts purchase on this code, because the failure is downstream of the sensor maybe 60 percent of the time on trucks.

Vehicle-specific patterns: 2007-2013 GM full-size trucks and SUVs (Silverado, Sierra, Tahoe, Yukon, Suburban) with the 5.3L or 6.0L are the textbook case, where road salt and water intrusion corrode the APP connector pins behind the kick panel and the fix is to cut the connector off, solder in new terminals, and seal with adhesive heat shrink. 2008-2012 Ford Escape and Mercury Mariner suffer pedal sensor internal wiper failure that genuinely requires a new pedal assembly. 2006-2010 Dodge Ram 1500 and 2500 commonly throw P2138 alongside P2122 or P2127 because of a chafed APP harness where it routes past the brake booster bracket. 2011-2015 Chevy Cruze 1.4T sees pedal-internal failure plus PCM software updates from GM that recalibrate the correlation threshold. Estimated repair: $90 to $450.