catalysts are reactivated by being subjected to fuel rich spikes after being exposed to high temperature excess oxygen conditions which typically arise during programmed fuel cut engine control strategies thus the present invention represents a departure from current control strategies by providing fuel rich spikes during engine control modes when conventional practice is not to provide rich fuel spikes application_number US 97671297 A description the following are definitions of terms useful in understanding the present invention stoichiometric ratio the mass of air or oxygen required to completely burn a unit mass of fuel to carbon dioxide and water with no oxygen left over lean burn condition a condition where the amount of air or oxygen is greater than the stoichiometric ratio thus this condition is characterized by having excess oxygen present after the fuel is burned e g 5 10 oxygen rich burn condition a condition where the amount of air or oxygen is less than the stoichiometric amount needed to combust the fuel i e a fuel rich environment or condition lambda ratio u03bb the ratio of the actual air to fuel a f ratio to the stoichiometric air to fuel ratio when u03bb gt 1 this refers to a lean condition when u03bb lt 1 this refers to a rich condition nox catalyst as used herein this term signifies a combined reduction.