My FX35 i-key keyfob is giving me problems. I dunno' if it's because of the bluetooth error or a weak battery in the fob. Figured I'd ask what type of battery it needs and start there. G35s use the same assembly and battery, anyways (minus the trunk button).

 

Check the battery, it should have a number on it.

For my Z battery, it's a CR2025

 

Finally found a screw driver thin enough to pop the fob. Sony 3V CR2025 it is.

 

change the battery and you're fine