I have a 04 C230 and get a trouble code (don't have it with me) that basically says the the thermostat is opening before the coolant temp is at the correct level (opening too soon). The thermostat is opening at the correct temperature and works correctely after that. I assume the sensor for the coolant on the engine side of the thermostat is bad. Where is it located? Or what else could cause this problem. I have to reset the trouble codes (clear it) and it stays off for a week or so and then is back on again.

More likely the thermostat is bad. What makes you think it is good?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I tested the thermostat and it tested good. I replaced it anyway. It is not the problem. The trouble code is not for a bad thermostat, but on that is opening too soon (not the problem here) or a bad sensor.