Key Fob Batteries and Programming: What You Can Do Yourself

When a car's remote starts to fail, most people assume the fob is broken. Usually it isn't. The small coin cell inside has simply run down, and changing it takes a few minutes. Programming is a separate job, needed when the car has to learn a new or replacement key. Knowing which one you're dealing with saves money and a trip.

Signs the battery is running down

• Shorter range. You have to walk closer to the car before lock and unlock work.
• Repeated presses. A button that used to work first time now needs two or three tries.
• No indicator light. Many fobs have a small light that blinks when you press a button; Mazda's CX-5 manual lists a transmitter light that doesn't blink as one sign of a weak battery.
• A warning on the dash. Some vehicles tell you directly. In the CX-5, Mazda says the green KEY indicator flashes for about 30 seconds after the engine is switched off when the key battery is low.

If you see these signs, try a new battery before anything else.

Which battery your fob takes

Most fobs run on a 3-volt lithium coin cell, and CR2032 and CR2025 are two of the most common sizes. Ford's 2012 Taurus Owner's Guide specifies one CR2032 or equivalent for its transmitters; Mazda's CX-5 manual specifies a CR2025 or equivalent. The two look almost identical, but in the standard coin-cell naming the last two digits give the thickness: a CR2032 is 3.2 mm thick and a CR2025 is 2.5 mm. Both are 20 mm across. Using the wrong one can mean a loose fit and poor contact.

Other vehicles use other sizes, so don't guess. The battery code is printed on the old cell, and your owner's manual will name it in the section on the key or remote.

How to change a key fob battery

Every fob opens a little differently, so check the steps in your owner's manual first. The general pattern is the same for most:

1. Remove the mechanical key. Smart keys usually hide a metal emergency key inside. Mazda and Ford both start by pulling it out, which exposes the slot used to open the case.
2. Open the case gently. Ford's manual uses a thin coin twisted in the slot; Mazda's uses a flathead screwdriver wrapped in tape so it doesn't mark the plastic.
3. Note which way the old battery faces before you take it out. Most show the positive (+) side, and the new one has to go in the same way. Mazda warns that a battery installed incorrectly can leak.
4. Fit the new battery without touching the circuit board. Mazda's manual says not to touch the internal circuits or bend the terminals. Ford's says not to wipe off any grease on the battery terminals.
5. Snap the case closed and refit any rubber gasket so it seals.
6. Test every button at the car before you throw the packaging away.
7. Dispose of the old cell properly. Ford asks owners to follow local regulations for transmitter batteries. Coin cells are dangerous if swallowed, so keep old and new ones away from children.

Mazda notes that having its dealer replace the battery is recommended to avoid damaging the key, while still giving owners the steps to do it themselves. If your fob is old, cracked or hard to open, that advice is worth taking, and we do fob batteries too.

Does a fob need programming after a battery change?

Usually not. Ford's owner's guide states that replacing the battery will not cause the transmitter or intelligent access key to become de-programmed from the vehicle. If a fob still won't work after a fresh battery, check that the battery is the right type and is facing the right way. If it still fails, the fob itself may be damaged, or the car may have lost its pairing, and that's when programming comes in.

When a fob really needs programming

• You've bought a replacement fob because one was lost, stolen or broken.
• You want an extra key for another driver.
• You bought a used car that came with only one key.
• A key was lost and you want it removed from the car's memory, so it can't be used to start the car.

Some manufacturers let owners program a key themselves under certain conditions. Ford's guide, for example, says you need at least two previously programmed transmitters to do it yourself; otherwise, take all transmitters to the dealer for reprogramming. Many newer vehicles need dedicated programming equipment, and the steps vary by make, model and year.

Cloning vs programming

These two words get used interchangeably, but they're different jobs.

Cloning copies the electronic identity of a working key onto a new key, so the car treats the copy as the original. It doesn't change anything in the car, and it needs a working key to copy from.

Programming teaches the car to recognize a new key, adding it to the list of keys the vehicle will accept. It's the way to go when there's no working key to copy, and it's also how a lost key gets removed from the car's memory.

Which one suits your vehicle depends on its system. We do both: cloning a working key, or programming a new key to the vehicle.

When to bring it to Cote Lock

Change the battery yourself if you're comfortable doing it. Bring it to us when:

• You'd rather not pry the case open, or you're not sure which battery it takes. We do fob batteries.
• You need a spare key or fob. We supply factory original automotive keys and fobs and duplicate keys.
• A new fob needs cloning or programming to your vehicle.

Car keys vary so much that we price them per vehicle. Call 519-894-7055 with the year, make and model, or request a quote. Our car keys and fobs page has more detail, and our locksmith cost guide lists published price ranges for fobs and smart keys.