Cool. your sig says 2010 so I thought it was Gen3 but just wanted to be sure. The Gen2 prius doesn't have the EV button as standard equipment in the US, but it does in some other countries, including Australia. Anyway I get exactly the same issue with my Gen2 and the EV button. It just stops working when the temperature is too hot. I think the EV button is considered as a very non-essential feature and shuts of even at fairly mild battery temperatures. Note that EV mode still works even when the EV button is disabled, as in the car will still drive under purely electric power at low speed or low power requirements as per usual. If however the battery really gets too really then the computer will just about completely inhibit running full EV operation at all, and the engine will run all the time. Hopefully you rarely or never get that condition.