Is it the battery or a dead fob?

Most remotes fail in a boring way: the battery is low. A smaller number have a worn button pad, a cracked case, or electronics that no longer talk to the car. Work through the cheap checks first.

What a weak battery usually looks like

You have to stand closer to the car than you used to.
The lock or unlock button needs two or three presses.
The remote works only after you hold it against the door handle or the start button.
The buttons still have a clear click. The plastic is not broken or swollen.

If the same car still unlocks with a second remote, the car’s receiver is probably fine. Focus on this fob.

Swap the coin cell

Look at the face of the old cell. The size is printed on it — common markings are CR2032, CR2025, and CR1632. Match the size exactly. Note which side is marked + before you lift the cell; in most trays the plus side faces the metal clip.

Use a plastic pry tool or a fingernail in the factory slot. Some cases have one small screw. After a swap, wait a minute and try the buttons from a few steps away.

When the fob itself is likely worn out

A new battery does not fix every remote.

The buttons feel mushy or do nothing after a confirmed fresh cell.
The case was cracked, left in a wet pocket, or washed with laundry.
The circuit board flexes when you press a button.

At that point you are shopping for a replacement remote, not another coin cell. For sticky pads and cracked shells that still have a good board, see Sticky buttons or a cracked key-fob case.

Honest limit

Do not pay for a full programmed key if a coin cell and ten minutes would have solved it. Late-model smart keys and push-button remotes often need a locksmith or dealer to pair — the owner’s manual section titled “remote” or “transmitter” is the check for what your car still allows at home.