The throttle position (TP) sensor is a potentiometer. The control module supplies the TP sensor a reference voltage, a signal, and ground circuits. When the throttle is depressed, the TP sensor signal rises to near the reference voltage. When the throttle is released, the TP sensor signal decreases from the reference voltage. The control module monitors the TP sensor signal circuit voltage in order to determine the throttle blade angle, or opening.

This DTC determines if the TP sensor signal input to the VCM does not closely match what the predicted value for the TP sensor should be. The VCM calculates what the TP signal voltage should be based on the engine RPM. If the TP sensor voltage and the predicted TP sensor values do not closely match, this DTC will be set.

The calculated throttle position is less than the actual throttle position for more than 5 seconds.

The VCM turns ON the MIL after 2 consecutive driving cycles with the fault active. The TP sensor reads 0% if the vehicle speed is less than 2 mph. The TP sensor reads 10% if the vehicle speed is greater than 2 mp/h. The scan tool will show a defaulted value.

The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.

The scan tool displays the throttle position in volts. It should display 0.20 volts to 0.90 volts with the throttle closed and ignition ON or at idle. The voltage should increase at a steady rate as the throttle is moved toward Wide Open Throttle (WOT). Also, the scan tool displays the throttle angle %. 0% = closed throttle. 100% = WOT.

Check the condition of the connector and sensor terminals for moisture or corrosion, and clean or replace as necessary. If corrosion is found, check the condition of the connector seal and repair or replace as necessary. If DTC P0121 is intermittent, refer to Symptoms .

The numbers below refer to the step numbers on the diagnostic table.

2. The TP sensor has an auto zeroing feature. If the voltage reading is within the range of about 0.2 to 0.9 volt, the VCM uses that value as a closed throttle. If the reading is out of the auto zero range at the closed throttle, check for a binding throttle or a damaged linkage. If the throttle and the linkage are okay, continue with the diagnosis.

4. With the TP sensor disconnected, the TP sensor voltage should go low if the VCM and the wiring are okay.