DTC P2121 Throttle / Pedal Position Sensor / Switch "D" Circuit Range / Performance: Fail-Safe
WARNING: This page is about a different car, the 2007 Toyota Land Cruiser. However, it is still accessible from the selected car via links, so may be relevant.
When any of DTCs P2120, P2121, P2122, P2123, P2125, P2127, P2128 and P2138 are set, the ECM enters fail-safe mode. If either of the 2 sensor circuits malfunction, the ECM uses the remaining circuit to calculate the accelerator pedal position to allow the vehicle to continue driving. If both of the circuits malfunction, the ECM regards the accelerator pedal as being released. As a result, the throttle valve is closed and the engine idles.
Fail-safe mode continues until a pass condition is detected, and the ignition switch is turned OFF.