While replacing coolant thermostat....

That photo is worth 1000 words (of thanks)

JsL wrote

There are 4 orings p/n 11537835497 that are $2.50 each that would be nice while you have it apart https://www.ecstuning.com/b-genuine-bmw-parts/o-ring/11537835497/

FWIW: I ordered that part number from BMW and got o rings that were too small.

How do you know if you need to replace the thermostat or just the sensor? I just got a 2B59 code.