Diagnosis & Repair
WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.
Using Toyota hand-held tester or scan tool, check for any other DTCs. If other DTCs exists, diagnose and repair those DTCs first and then retest. If only DTC P0121 exists, replace TP sensor. See THROTTLE BODY under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article.