My Apple Bluetooth wireless keyboard and mouse randomly disconnect and reconnect to my 12-inch MacBook (early 2015). My local Genius Bar has been unable to fix this despite replacing the MacBook’s motherboard. I can’t replicate the problem using the same keyboard and mouse with another Mac. What’s wrong?
This is a not uncommon problem in El Capitan and Sierra. A useful preliminary check is to disconnect all other peripherals apart from any wired network connection, then start up with your keyboard and mouse connected using their charging cables, or using wired keyboard and mouse, holding to enter Apple Diagnostics. Ensure your MacBook is well ventilated, to help keep it cool. Chances are that those tests will return a normal result of “ADP000,” indicating that your Bluetooth hardware is working normally, and no issues were detected with the keyboard or mouse.
You should find Bluetooth signal propagation best when your MacBook, keyboard, and mouse are all on the same flat surface without intervening objects such as paper or china. It’s worth checking this, although it’s unlikely to be the cause of the problem.
Remaining Bluetooth dropouts are then almost certainly the result of these long-known problems in macOS, and hopefully will one day be fixed in an update.