Keep the three blue dots in the middle. Mine has the same problem. If it goes to two dots, A/C temps don't go to <40 degrees. I'm going to wedge something in it or use black tape. I have a new heater control valve to install. I thought it was the problem until I turned the knob to three blue dots. Since I have it, I might as well install it. 150k+ miles is a lot of use. Winter is coming too.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)