P0121

P0121 means the Throttle Position Sensor signal is electrically in range but does not track throttle input correctly (the signal lags, jumps, or sits at a flat spot through part of the pedal travel). On a 3-wire potentiometer TPS, the ECM expects a smooth voltage sweep from roughly 0.5V at closed throttle to 4.5V at WOT; any flat spot, dropout, or non-linear region inside that sweep sets P0121. On a 6-wire electronic throttle body (ETC) with redundant TPS1/TPS2 channels, the ECM compares the two channels and sets P0121 when they disagree by more than the calibrated tolerance (usually 5-10% across the sweep). Cheapest-first ladder: scan live TPS data with the engine off and key on, then slowly sweep the pedal from idle to WOT. A healthy TPS shows a perfectly smooth voltage climb from 0.5V to 4.5V with no flat spots; a worn potentiometer shows dropouts at specific positions (commonly 15-25% throttle where the wiper has worn a groove from years of idle-cruise driving). On an ETC, both channels must track in opposite directions on most platforms (TPS1 goes 0.5V to 4.5V while TPS2 goes 4.5V to 0.5V); a divergence over 0.3V at any point sets P0121. Do not replace the throttle body before cleaning the carbon off the throttle plate, because a stuck or sticky plate on a drive-by-cable system can stick at an angle that the TPS reads as a flat spot.

Vehicle-specific patterns: 1996-2003 Ford Explorer/Ranger 4.0L SOHC throws P0121 routinely past 100k miles from potentiometer wear at the cruise position; the throttle body assembly is the replacement on that platform because the TPS is not serviced separately. 2002-2008 Dodge Ram 4.7L V8 sees P0121 from carbon buildup on the throttle plate causing a mechanical sticking at the 10-15% position that the TPS reads as a non-linearity. 2005-2010 Chevrolet Cobalt/HHR 2.2L Ecotec sees P0121 from a TSB-acknowledged TPS dropout at low throttle angles, GM bulletin extended warranty coverage on affected VINs. 2007-2013 BMW N52/N54 sees P0121 from internal ETC failure where TPS1 and TPS2 drift apart at part-throttle; the entire throttle housing is the BMW replacement. Estimated repair: $80 to $600.