Throttle/Pedal Position Sensor/Switch "D" Circuit Range/Performance
OBD-II code P2121 indicates a problem with the accelerator pedal position, or APP, sensor.
The accelerator pedal position sensor is located on the accelerator pedal bracket. This pedal position sensor has two sensor elements and two signal outputs which are VPA & VPA2. VPA is there to detect problems in the VPA. When there are differences in the voltage outputs of the VPA and VPA2 that are different from the standard, the ECM will determine that the accelerator pedal position sensor is not functioning properly.
There is only one common cause of the P2121 trouble code and that's the accelerator position sensor itself. In many applications, this sensor is in need of replacement. Other symptoms of a bad accelerator pedal position sensor include erratic throttle response and a pedal that will go "dead" while there is power to your vehicle. In very rare circumstances, any trouble codes that won't go away, even after faulty parts are replaced, could indicate problems with the engine control module, or ECM. Problems with the ECM include poor performance and erratic shifting for automatic transmissions.
For a comprehensive list of trouble codes, check out this article on the Toyota Parts Center Blog.