Replace BMW Key Battery It could be the time to change your battery in the event that your BMW key won't allow you to open your car or turn on the engine. It's a simple fix. Simply remove the key fob, replace the battery, and then gently put it back. https://car-key-replacement61857.jts-blog.com/29472442/is-bmw-key-fobs-really-as-vital-as-everyone-says