I looked briefly at the first two answers, Randy and Aye Roll.

Both of them are exactly what I'd say as well; the temperature compensating board part of the temp gauge is most likely faulty. The easiest way to find out is to give the instrument cluster a sharp tap right near the temperature gauge. Tap the cluster itself, not the dashboard that overhangs the cluster because the dash is padded and won't transfer the bump.

Usually, that causes a marked change within a minute or two. Generally it will either go way down, or it will level itself at the car's actual temperature. Don't get carried away, though--if it works once or twice, you've diagnosed a bad temperature circuit board. If it doesn't work, there MIGHT be a cause elsewhere, but it still sounds like a classic case of "tried everything and it STILL overheats. Don't beat up the cluster--get the circuit board replaced. (I once broke a temp gauge when it spiked with a bogus overheat, because I really banged on it.)

The most common cause of 'overheating' is the temp board. We had a whole bag of new circuit boards at the shop, for exactly that reason. Not only that, most of the cars on the dealer's lot needed it too.
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Chris Herbst, near Chicago.