It also can check to see if the ignition system is working ok. As a matter of fact, when trying to fix a driveability problem, most good technicians will take a look at the vehicles tail pipe readings first. These readings will give them direction. When you fail because of high limits of HC or hydrocarbons, it means there is raw fuel that is being sent out the tailpipe because your engine isn't converting the fuel to energy. The raw fuel actually washes the protective layer of oil off moving parts inside your engine and contributes to engine wear which can lead to premature failure.
Some common causes are spark plugs, spark plug wires, misadjusted timing or vacuum leaks.
Most of the common causes relate to the carburetor or the fuel delivery system. A CO failure will always be rooted in the carburetor or fuel injection system. CO failures are not fixed with spark plugs. If there is too much fuel and not enough air, a CO failure is usually seen. When there is too much air and not enough fuel, a HC failure is generally seen.