When does the key fob battery need replacing?

A key fob has a little battery that powers the electronic chip inside the fob. The chip works as a transmitter/receiver and stores the ID code that is registered to the car. In modern cars with a push button start, a mechanical key might be hidden inside the fob. This key will open the driver's door in case the fob battery is dead or the main car battery is discharged.
Battery and door key inside the key fob Battery and door key inside the key fob.

In most cars, a fob battery is not rechargeable, and must be replaced when it gets low.

What are the symptoms of a weak battery in the fob? As the battery voltage drops, the transmitter loses its range, which means you have to be closer to the car for the fob to work. In cars with a push button start, a vehicle may not always recognize the smart key fob in your pocket or purse if the fob battery is getting weaker. In this case, you might have to hold the fob closer to the "Start" button or insert it into a special slot to start the engine. In some cars, a "key fob battery is low" message may also pop up on the instrument panel.

In some cars, a key fob has a little LED light that comes on when pressing any of the buttons. If the LED light doesn't come on at all, the fob battery is dead or there is some other problem with the fob. If you already have the fob battery out, you can check the battery voltage with a voltmeter.

Checking the key fob battery voltage This battery is bad, as it shows only 2.858 Volts. A good 3-Volt battery should show more than 3 Volts.
If a 3-Volt battery shows less than 3 Volts, it's weak. This CR2032 battery in the photo shows just over 2.8 Volts; it must be replaced.

How long does the key fob battery last? Typically a fob battery lasts for up to three-four years, although it is not uncommon for the battery to fail after one year. A fob battery might need replacing sooner if, for example, moisture gets inside the fob or if one of the buttons is sticking.

What kind of battery does a key fob take? Different cars use different batteries. The most common types are CR2025 and CR2032 3-Volt batteries. You can find the exact type in the owner's manual. For example, according to the 2017 Ford Explorer owner's manual, the Intelligent Access key fob with push-button start takes two CR2025 batteries. Batteries like this are sold in any electronics store.

How easy is it to change a key fob battery? In some cars, it's easy, in others it's a bit trickier. Check the owner's manual for directions and precautions. There are plenty of Youtube videos on how to do it. You might need a small flat or Phillips screwdriver.
For example, to open the Mazda key fob in the photo at the top of the page, we pried it open with a little flat screwdriver. Be careful; in some cars the plastic tabs that hold the parts of the key fob together break easily. In some cars, there could be some tiny parts that is easy to loose.

When replacing the battery, watch for the way it's installed. One side of the battery is marked with the "+" sign. It's important to install the new battery the same way. If you are not mechanically inclined, visit your dealer; they should be able to change the key fob battery for you. Typically it doesn't take more than a few minutes. We checked with several dealers and prices we got were within $10. A key fob doesn't need any programming if only a fob battery is replaced.