I skimmed through all the messages and didn't see (or maybe missed) another person pointing to this reason that EV mode not available appearing.. There are more things at play here than just the engine temperature and cabin heating. As stated in the manual, our cars are not on usually long enough to draw in and burn the excess vapors in the gas tank - hence we have a pressurized gas tank.. It could be a function to draw all the vapors out of the carbon canister then resume normal functionality.. The second theory I have is that our batteries need to be balanced or equalized (all cells brought to the same voltage).. This would take the engine running and charging the batteries to a specified voltage every so often.. I wish we had the service manual available to check out the behaviours we experience. The owners manual does a crappy job informing us..