If your vehicle is showing key fob won't unlock doors even after replacing its battery, here are the most likely causes and the parts you'll need to fix each one.
The new coin battery was accidentally installed upside down (reversed polarity) or the battery contacts were bent during installation, preventing the remote from getting power.
The remote lost its synchronization with the vehicle's security system. This frequently happens if the fob is left without a battery for too long during replacement.
The internal circuit board, button contacts, or solder joints inside the fob are broken or worn out from age, drops, or moisture. A new battery won't fix a physically broken circuit.
The car's main 12V battery is completely dead. The key fob is sending the unlock signal, but the vehicle has no electrical power to actuate the door locks.
The vehicle's internal antenna or body control module that receives the fob's signal has failed, lost its programming, or has a blown fuse.