If Code p1121 appears in a 2007 Prius, what is the problem?

On a 2007 Toyota Prius, diagnostic fault code P1121 indicates that the electric throttle control actuator has failed. If You're receiving this code, You'll typically have an illuminated check engine light, and very likely no throttle response to pushing the accelerator pedal. This is, obviously, not a very good situation to be in. The throttle control actuator consists of the throttle control motor, the throttle position sensor, and others. This is operated by the ECM (engine control module), and it's the system by which the throttle valve is opened and closed. You'll need a digital multimeter to precisely diagnose what Your issue may be; though it's possible to replace the entire apparatus, You may save a few dollars by isolating Your failed component.