There’s a secret feature in your car that your mechanic has done his best to keep a secret. Every car sold in the U.S. since 1996 features a built-in engine control computer that can be accessed with the right tools. This is called On Board Diagnostics-II (OBD-II), and it’s usually used as a way for mechanics to diagnose problems. But there’s no reason that you can’t use it as a way to look inside your engine and figure out what’s broken, what isn’t, and help and keep it running at peak performance. In other words, you can learn to hack into your car’s computer.