Are you sure the engine is actually getting up to temperature? If so, and if the temperature sensor checks out, the problem could be the connections between the gauge itself and the SI board. I would suspect the threaded connector that's visible from the back of the instrument cluster. This connector has given me a lot of grief. Undo the nut on the back, clean everything you see with a wire brush, and reassemble. The fuel gauge has a similar connector. You might as well clean it if you're back there. My engine temp gauge is broken. It is always in the blue zone. All other gauges work fine. What could be at fault (suspect thermostat)? Is it something easy to fix/install?