BMW X5 3.0 Thermostat?

[lebahawk]

Hello, I have a quick question. How can I tell if the Thermostat is good or bad ? is there a quick way to figure out ? here is why.
A while ago 2 months ago the coolant temp was reading extremly high (Needle on red) as soon as I start the car. After I started driving on the freeway it went down to the cold (Needle near the blue)
Last week, weather changed and it warmed up, so I was running the AC coolant temp was at vertical were it needs to be. But I got a service engine soon light. I checked the code and it was P0126 (thermostat), so I cleared the code and been driving it. Today I drove around 100 miles back and forth with the AC running high and low speed and the needle stayed very much at vertical. No light came on or anything sofar. what do you suggest ? shall I replace the thermostat anyway ? or shall i wait and see ? vehicle has about 120K miles.