P0121

CIRCUIT DESCRIPTION
The Throttle Position (TP) sensor provides a voltage signal that changes relative to the throttle opening. The TP sensor signal voltage varies from about 0.6 volts at idle to about 4.8 volts at Wide Open Throttle (WOT).

The TP sensor signal is one of the most important inputs used by the VCM for fuel control and for most of the VCM control inputs. This DTC is a type B DTC.

CONDITIONS FOR SETTING THE DTC
The following conditions will set the DTC:

^ No MAP DTCs are present.
^ Engine is running.
^ MAP less than 60 kPa.
^ Change in TP is less than 2%.
^ The last throttle position is greater than the calculated throttle position based on engine RPM.

ACTION TAKEN WHEN THE DTC SETS
The VCM turns on the Malfunction Indicator Lamp (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 vehicle speed is greater than 2 mph. The scan tool will show defaulted value.

CONDITIONS FOR CLEARING THE MIL/DTC
The VCM turns the MIL off 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 (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

DIAGNOSTIC AIDS
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. Clean the connector and sensor terminals for moisture or corrosion. Replace the connector and sensor terminals for moisture or corrosion as necessary. If corrosion is found, check the condition of the connector seal, and repair or replace the connector seal as necessary. If DTC P0121 is intermittent, refer to the Driveability Symptom Tables.

TEST DESCRIPTION
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.