Method and system for monitoring the operation of catalytic converters of an internal combustion engine

The invention concerns a method for monitoring the operating conditions of at least two catalytic converters of a heat engine operating in lean burn, characterised in that it consists in: assessing the efficacy of hydrocarbon treatment by the so-called priming catalytic converter (5) with low oxygen absorption capacity and close to the engine; assessing the oxygen absorption capacity by the so-called nitrogen oxide storage catalytic converter (6), at each change of richness shifting from a lean burn to a rich or stoichiometric mixture, for purging the catalytic converter of nitrogen oxides; comparing the result of said two simultaneous and separate assessments with a reference curve (CHC) representing the limit of the set of possible combinations of the priming catalytic converter and the storage catalytic converter deemed to be satisfactory for treating hydrocarbons. The invention is applicable to motor vehicles.