Have a '98 Deville w/ 117k mi. -- the Outside temperature gauge shows 133 degrees, when it should read below 40. And of course, if it thinks it over 100 degrees outside, the heater blows cold air unless I boost it up to 90, then it blows 'warm' air. I checked the coolant level, a bit low but shouldn't be a factor. Just started happening a couple days ago, any thoughts? Thanks.