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Device placed within the exhaust system used to catalyze pollutant gasses found in exhaust gasses. Is normally located after the exhaust header, but before the muffler. Can be configured as 2-way or a 3-way: the 2-way affecting hydro-carbons (HC) and Carbon-monoxide (CO) gasses; the 3-way affects Nitrogen Oxide (NOx - do not confuse with Nitrous Oxide or NO2) as well.

Catalysts work best when fuel mixture is carefully controlled very near stoichiometric(Lambda 1.000). In lean mixtures NOx is poorly reduced due lack of available fuels. In rich mixtures fuels are not completely burned due to lack of available Oxygen.

Catalyst failure has two primary causes. 1) Heat damage usually caused by poor control of the fuel amount and/or mixture fed to the catalyst. 2) Damage from contaminates. Phosphorus is the most common permanent contaminate. It comes from engine oil and coolant. It combines with the catalyst substrate and cannot be removed. Sulfur is another contaminate that is common. It is less of a problem because it can be removed by running the system with a slightly rich mixture. Lead is much less common but very aggressive contaminate.