Method and system for maintaining efficiency of a three-way catalyst

A system comprising:

• a gasoline-fuelled internal combustion engine, wherein the engine operates at substantially stoichiometric conditions during normal operation;
• a three-way catalyst (TWC) mounted under-floor and arranged in an exhaust passage to have exhaust gases passed continuously therethrough;
• and
• an engine management system, wherein the engine management system is programmed to initiate a condition of the system in which the engine exhaust gases are enriched with at least one of CO and hydrocarbons during cold start when the engine is operated under rich conditions and under any driving conditions, and secondary air is added to the engine exhaust gases in the exhaust passage upstream of the underfloor TWC to provide lean conditions over the underfloor TWC lean and to cause an exotherm by oxidation of the at least one of CO and hydrocarbons over the underfloor TWC thereby to raise the temperature of the underfloor TWC to at least 550° C., whereby sulphur poisoning of the underfloor TWC is reduced, during at least one portion of every operation of the engine.