Possible Cause
WARNING: This page is about a different car, the 2003 Toyota 4Runner. 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 when 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. When this occurs, throttle valve is controlled by throttle cable connected to accelerator pedal. Possible cause is defective TP sensor.