2000 Lumina, heat gauge problem or sensor problem

I have 2000 lumina giving me a little trouble after 85,000 miles. Heat gauge only registers about 1/6 when outside abient is around 45. when it gets colder, the temp gauge barely moves off the cold peg. I put an electronic temp tester (infared light) on the t-stat and it showed 185 degrees which tells me the t-stat is opening. So.. is my sensor bad, or could it still be t-stat regardless of what I am thinking. Also, is the 2000 lumina an ok car. So far it has been, but with the miles I am thinking of trading it or buying new tires. which one?