The flawless functioning of a three-way catalyser requires a certain air-fuel mixture: ideally all components are completely burned when 14.7 parts of air are mixed with one part of fuel.
In this case, lambda is equal to one. The Greek letter lambda stands for the so-called "air ratio". Only when lambda = 1 can the catalyser optimally fulfill its task of cleaning the exhaust gas.
The lambda control forms a closed control loop in which the lambda sensor, working as a measuring sensor in the exhaust flow, measures its composition. In the event of deviations, regulation via the electronic engine management system ensures that the injection system meters the fuel quantity differently in relation to the proportion of air.