method of diagnosing an oxidation function lean

Method and diagnostic apparatus according to the operating state of a catalytic element (2) of an engine exhaust line (1) for internal combustion characterized in that it consists of comparing a reference value output by a first oxygen sensor (S1) disposed upstream of a catalytic element to a value measured continuously during running of the vehicle with a second sensor (S2) to oxygen placed downstream of the catalytic element. The method comprising a step of evaluating a difference obtained in the comparison between the reference value and the measured value.