This is an example of a converter with carbon deposits in the ceramic catalyst. This is usually a result of oil or antifreeze entering the exhaust system or a too-rich fuel mixture. The heavy carbon deposit clogs the converter and reduces exhaust flow. This increases backpressure and causes the entire exhaust system to heat up. The heat backs up into the engine compartment and may result in a number of heat related engine problems. Mild carbon scoring may do less damage to engine components but it may seriously affect the converter's ability to reduce harmful emissions. It could easily cause a vehicle to fail an emission test. Carbon deposits may be the result of faulty valves, worn piston rings, worn or leaking gaskets, or lead in the fuel.