Possible Causes
WARNING: This page is about a different car, the 2003 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.
This DTC uses two-trip detection logic. DTC is set after vehicle speed exceeds 19 MPH and improper input signals from TP sensor are received at ECM. If DTC P0121 exists, ECM will turn off the power to throttle control motor and throttle valve is closed by the return spring. Possible cause is defective TP sensor or defective ECM.