DTC P0121: TP Sensor No. 1 Circuit Performance: Description

The Throttle Position (TP) sensors 1 and 2 are located within the throttle body assembly. The TP sensors share a common 5-volt reference circuit and a common low reference circuit. Each TP sensor has an individual signal circuit which provides the Engine Control Module (ECM) with a signal voltage proportional to the throttle plate movement. When the throttle plate is in the closed position, the TP sensor 1 signal voltage is near the low reference and increases as the throttle plate is opened. The TP sensor 2 signal voltage at closed throttle is near the 5-volt reference and decreases as the throttle plate is opened. The ECM compares the signal of the TP sensors to the Mass Airflow (MAF) sensor when the engine is running to determine if the sensor readings are correct. The control module also compares the signal of TP sensor 1 and TP sensor 2 through the entire range. If the ECM detects a difference of more than 6 percent between sensor 1 and sensor 2, and a difference from the predicted range by more than 9 percent, DTC P0121 will set.