when you say stuck on defrost, are you referring to the **** itself or that no matter what you dial up, air only comes out of the defrost vents?

 

 

check out volvotips.com for some diagnostics suggestions. Any ECC fault codes like a blinking AC or REC light etc? Could be the blend door motor may have disconnected from the rod to the door. can you hear the motors actuate when you change air flow selections?

 

 

volvotips.com has a 850 diagostics page that may help. The ECC code will tell you if the motor is the issue.