DTC P2420: Evaporative Emission Pressure Switching Valve Stuck OFF [06/2014 - 08/2015]: Description
WARNING: This page is about a different car, the 2015 Lexus IS 250 and 2014 Lexus IS 250. However, it is still accessible from the selected car via links, so may be relevant.
The description can be found in EVAP (Evaporative Emission) System.
Refer to EVAP System [04/2013 - 08/2015]