I have had a couple of sticky sensors. I think there is a two sensors under back seat. One seems to do full to half and the other does half to empty - but this is a complete guess, dont take my word for it. I find that just filling up unsticks the sensors as the fuel sloshes about, i really cant believe the grade makes a difference. Also, fill up to about 3/4 so you have more than half a tank. Finally, yes you can access them by removing back seat, it only takes about 5 minutes as I have done it.

BTW, this is the best way to remove the wrong fuel from tank!!!