Key fob not working after a battery change

Most “it stopped working after I changed the battery” problems come down to the cell itself or how it sits. Work through these in order before paying a dealer.

1. Wrong size code. Compare the new cell with the old one, digit for digit. CR2025 vs CR2032 (and CR1616 vs CR1620) look identical from the top but differ in thickness. Find your vehicle on the make list.
2. Upside down. Match the + side to the marking in the holder. Energizer's own instructions stress following the + and − symbols.
3. Contacts not touching. Look for bent or flattened metal tabs. Clean the contact surfaces gently with a clean pencil eraser or cloth (Energizer's advice) and make sure the cell sits flat.
4. Coated or textured cells. Some retail cells carry a bitter child-deterrent coating on one face. Owners report that some fobs make poor contact with these or won't close properly. If the fault started with a new brand, try a cell without the coating.
5. Old or weak “new” battery. Coin cells lose charge in storage. Check the expiry date on the pack, and try a second fresh cell before assuming the fob is faulty.
6. Low-battery warning still showing. Owners report this on several makes. Check the owner's manual for a fob power-saving mode (some Mazda owners mention one) or a step to clear the message. If it persists with a confirmed-good cell of the right code, the fob or the car may need diagnosis.
7. Both fobs, or the car's battery? If neither fob works, the problem may be the vehicle's 12-volt battery or receiver rather than the fob.
8. Reprogramming. A battery change normally doesn't erase pairing. If your owner's manual describes a resync step for your model, follow it; otherwise a dealer or locksmith can test the fob.