Throttle Position Sensor

The Throttle Position (TP) sensor is a potentiometer connected to throttle shaft of throttle body. The TP sensor electrical circuit consists of a 5 volt supply line and a ground line, both provided by powertrain control module (PCM)/engine control module (ECM). The PCM/ECM calculates throttle position by monitoring voltage on this signal line. The TP sensor output changes as accelerator pedal is moved, changing throttle valve angle. At a closed throttle position, output of TP sensor is low, about 0.5 volt. As throttle valve opens, output increases so that, at Wide Open Throttle (WOT), tie output voltage will be about 5 volts.

The PCM/ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent bursts of fuel from injector and an unstable idle, because PCM/ECM thinks throttle is moving. A problem in any of TP sensor circuits should set a diagnostic trouble code (DTC) P0121 or P0122. Once DTC is set, PCM/ECM will substitute a default value for TP sensor and some vehicle performance will return. A DTC P0121 will cause a high idle speed.