what side is blowing hot? change it on that side.

once the old one is off, work the knob by hand to make sure thats the issue.

if warm on the driver side it could be low on freon. turning the knob to hot or cold will tell you. it only moves a little bit, dont go cranking on it.

if you had the battery off or it went dead these things can freak out and need to be reset. my regal i unplug my climate control fuse for a minute then put it back in and it resets and works fine again.