The main reason these days is PNP switch BUT years ago when 850s were new, all that got that code use to get 2 solenoids (S1,S2) replaced and those are attached to the valve body. To replace those, one needs to lower the left side of the subframe and remove the trans pan (front of trans).
In my years of experience, I have never replaced a trans because of that code in itself UNLESS there's more to the story, like it's slipping and noisy.
Assuming it still shifts OK when not in "limp home mode" (codes cleared), try a new PNP switch and trans fluid flush if the fluid's dirty before going further.
On the other hand, if it's slipping and making noise, it's likely to need a reman trans but that job's only $2,350 at our US dealership. The trans runs about $1600 and 10 hours labor.