The document discusses how to identify a hole in a car muffler. A muffler is used to lower exhaust noise and allow exhaust gases to flow from the engine. Over time, driving conditions like potholes or icy roads can wear the muffler and cause holes. Signs of a muffler hole include producing carbon monoxide in the cabin, rust around exhaust pipes, engine misfiring, failing an emission test, and unusual noises from the muffler getting louder. The document recommends inspecting the muffler if any of these signs are present and consulting a mechanic to address any holes.