Changing the battery in a current Mazda 3’s key fob is a simple job once you know the process. Start by locating the small button on the back of the key near the silver metal key ring lug, and depressing it. You can then tug the metal key ring lug out and pull the manual accessory key out of the key fob.
Take a flat-bladed screwdriver and twist it gently in the slots where the manual accessory key is located. This starts separating the two halves of the key fob, allowing you access to the battery, though you will need to run an old credit or gift card down each side of the key fob to fully separate the halves.
Once you have the case open you will see the thin, round Panasonic CR2025 battery which has a rubber weather-proofing cover. Once you’ve removed the old battery and switched the cover to your new battery, you can start reassembly by putting the new battery in the key shell, with the positive side facing up.
Snap the key fob halves back together, push the accessory key back in, and then test to make sure the keyless entry works on your car.
It is important to note that this guide is for the current fob-style key and the process will be different for earlier pop-out keys.