If the annoying Check Engine light causes your skin to crawl and requires a costly trip to the dealer to figure out what's up, it's also an opportunity for a digital home mechanic to take a sneak peek under the hood. Onboard diagnostics, or OBD-II, is how the car's electronics communicate with the world. It lets you look into dozens of the car's components, sensors, and microcontrollers to see what is working and what isn't. From the engine, transmission, and brakes to the air bags, steering, and air conditioning, it's all there. Some cars can even tell you if they've been driven faster than the recommended top speed.