Your exhaust emission system starts at the front of your car, right behind your engine. Exhaust manifolds connect to the cylinder heads, where they collect exhaust gases. As the gases pass through the exhaust system, they're analyzed by oxygen sensors, then refined by the catalytic converter and finally, muffled by the muffler, making your car run smoother, cleaner and quieter. It's a pretty complex system, and when there's a problem, it can be noisy and embarrassing.