DTC P2121 Throttle / Pedal Position Sensor / Switch "D" Circuit Range / Performance: Fail-Safe
WARNING: This page is about a different car, the 2008 Lexus IS F. However, it is still accessible from the selected car via links, so may be relevant.
The accelerator pedal position sensor has two (main and sub) sensor circuits If a malfunction occurs in either of the sensor circuits, the ECM detects the abnormal signal voltage difference between the two sensor circuits and switches to limp mode. In limp mode, the functioning circuit is used to calculate the accelerator pedal opening angle to allow the vehicle to continue driving If both circuits malfunction, the ECM regards the opening angle of the accelerator pedal as being fully closed. In this case, the throttle valve remains closed as if the engine is idling
If a pass condition is detected and then the engine switch is turned off, the fail safe operation stops and the system returns to a normal condition