Neither work. Same issue. I replaced the battery the first time this happened and it didn't do anything. There is only one battery for the key fob.Both keys fobs do not work? I can't remember but I think the C30 takes two batteries. Did you replace both?
What was the outcome on yours? Two out of three fobs on my S40 stopped working with the latest incident being yesterday, now I'm down to one. Apparently these are unnecessary expenses that Volvo is shoving down our throats.