Well... answering your inquiry about the "check gas cap" was bad karma for me. Would you believe the very next day after I responded to you, my ___n "check gas cap" message appeared for the first time in a year.
Now I feel foolish, but at least the NO OVERFILL procedure kept the light off for a year anyway. It came on immediately after I CAREFULLY filled my tank and removed the nozzle as soon as the pump cut off (that'* how I always do it and that has worked every time, until now). Well the think has come on with every start for the last 4 days. I am as stumped as you are now! Tonight I banded a sheet of rubber tightly over the fill neck to test my theory that MY GAS CAP HAS NOTHING TO DO WITH THE MESSAGE.
If you find out your problem please let me know. I'll do the same.
PS- You DO know that you can clear the message until the next startup by depressing the "Select" button don't you?