P0123

P0123 means the TPS signal voltage is above the ceiling, typically over 4.8V on a 3-wire potentiometer or over the TPS1 maximum on a 6-wire ETC. The ECM reads that as a WOT signal that physically cannot occur at the actual pedal position, which means the sensor has failed shorted to its 5V reference, the signal wire has shorted to a power rail in the harness, or the sensor ground has been lost (which pulls the signal up to the reference). On most platforms this drops the engine into severe reduced-power limp mode immediately because the ECM cannot tell a real WOT command from a faulted signal. Cheapest-first ladder: with key-on engine-off, scan-tool TPS voltage should sit at the closed-throttle calibrated rest (0.5-0.8V on most potentiometer TPS). If TPS reads 4.9V at idle with the pedal untouched, that is the P0123 signature. Unplug the connector and verify the signal wire drops to 0V with the sensor disconnected (open-circuit pulled to ground by the ECM); if it stays at 5V with the sensor unplugged, the harness is shorted to a 5V or 12V power source between the connector and the ECM. Verify the sensor ground is intact with under 0.1V drop, because a lost sensor ground will float the signal to reference voltage and look identical to a shorted sensor. Don't replace the ETC throttle body before checking the ground at the connector, because a corroded ground pin is the most common P0123 cause on Ford and GM trucks and cleaning the pin saves a $400 part.

Vehicle-specific patterns: 2004-2010 Ford F-150 5.4L 3V sees P0123 from a corroded ground pin in the ETC throttle body connector after water intrusion through the cowl drain, cleaning the connector and applying dielectric grease often clears it without replacement. 2007-2013 GM Silverado/Tahoe 5.3L sees P0123 from internal ETC failure where TPS1 shorts to 5V reference internally; the throttle body must be replaced and idle relearned. 2008-2014 Chrysler 300/Charger 5.7L Hemi sees P0123 from TIPM-related power distribution faults that feed back-EMF onto the TPS 5V reference, a TSB pattern. 2009-2016 Subaru Legacy/Outback 2.5L sees P0123 from the TPS1 element failing open in a way that pulls the signal to the 5V reference rail. Estimated repair: $80 to $550.