Catalyst poisoning

Catalytic converters become ineffective in the presence of lead due to catalyst poisoning, and the introduction of catalytic converters triggered the end of leaded gasoline. Catalyst poisoning occurs when a substance in the engine exhaust coats the surface of the catalyst, preventing further exhaust access to the catalytic materials.

Poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time to raise exhaust gas temperature, which may cause liquefaction or sublimation of the catalyst poison. Common catalyst poisons are lead, sulfur, zinc, manganese, silicone, and phosphorus; Zn, P, and S originate from lubricant antiwear additives such as ZDDP; sulfur and manganese primarily originate from fuel impurities or additives, respectively.

Silicone poisoning is usually the result of engine damage, such as a faulty cylinder head gasket or cracked casting, admitting silicate-containing coolant into the combustion chamber. Removal of sulfur from a catalyst surface by running heated exhaust gasses over the catalyst surface is often successful, however removal of lead deposits is often not possible (the lead becomes vaporized in the combustion chamber of a gasoline 4 stroke engine under the ambient temperature and pressure after charge air ignition, and condenses on the cooler catalytic converter core surface.

In particularly bad cases of catalyst poisoning by lead, the converter can actually become completely plugged with lead residue). Theoretically, catalyst poisoning could also occur if the charge air was contaminated by a catalyst poison, however catalyst poisons are all substances that are solid at the internal temperature of the catalytic converter, and thus precipitate out of the air.

Any condition that increases the concentration of CO or HC reaching the catalyst can cause it to overheat and melt down, restricting the exhaust flow, rendering the converter useless for emission control purposes, and creating an undercar fire hazard. Some such conditions are oil-burning engines, overly rich fuel mixtures, and misfires.