DTC P2121: Throttle / Pedal Position Sensor / Switch "D" Circuit Range / Performance [09/2014 - 11/2017]: Fail-Safe
WARNING: This page is about a different car, the 2017 Lexus RC 350, 2017 Lexus RC 300, 2016 Lexus RC 350, 2016 Lexus RC 300, and 2015 Lexus RC 350. However, it is still accessible from the selected car via links, so may be relevant.
When DTC P2120, P2121, P2122, P2123, P2125, P2127, P2128 or P2138 is stored, the ECM enters fail-safe mode. If either of the 2 sensor circuits malfunctions, the ECM limits the engine output. 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.
If a pass condition is detected and then the engine switch is turned off, the fail-safe operation stops and the system returns to normal.