Circuit Description

The DTC P0121 Throttle Position (TP) Sensor Performance diagnostic monitors TP sensor activity. TP sensor is a potentiometer whose resistance value changes along with throttle valve position. Powertrain Control Module (PCM) provides a 5-volt reference voltage to TP sensor. PCM reads voltage across TP sensor and uses voltage to calculate percentage of throttle valve opening. TP sensor resistance decreases as throttle valve opening increases. When TP sensor resistance decreases, voltage being monitored by PCM increases. TP sensor resistance increases when throttle valve opening decreases, therefore voltage being monitored by PCM will decrease. See TP SENSOR OPERATION  table.