1. Shorter range
You have to walk closer to the car, or press the button several times, before it locks or unlocks.
2. A warning on the dashboard
Many cars show a “key battery low” message or a key symbol. Replace the battery within a few weeks.
3. The car can’t find the key
Keyless entry or push-button start stops working, or works only some of the time.
4. Worse in cold weather
Cold lowers a coin battery’s voltage while it stays cold, so a weak one often fails first on a freezing morning. Warm the key in your hand or pocket and try again. If it works when warm but not when cold, replace the battery, and look up your car on this site to find the right one. In freezing weather, don’t leave the key in the car overnight.
5. If it dies completely
Use the metal key hidden inside the fob to open the door. On most cars with push-button start, holding the fob against the start button lets you start the engine. Your manual shows the exact spot.