Method for diagnosing a catalytic converter of an exhaust gas system with which an exhaust line of a controlled ignition combustion engine is equipped, the said system being of the type comprising a first and a second oxygen sensor installed respectively upstream and downstream of the said catalytic converter in the said exhaust line, in which method a richness-regulating setpoint able to adjust the richness of the engine according to the signal supplied by the upstream probe so as to maintain a stoichiometric mixture at the inlet of the catalytic converter is supplied, the said method comprising steps during the course of which, during diagnostics: - a regulation oscillation is applied to the said regulation setpoint and able alternately to impose a rich mixture and a lean mixture upstream of the catalytic converter and the amplitude of which is calibrated so as to keep the value of the mixture within the range of values of the catalytic window during which the efficiency of the catalytic converter is at a maximum; - a variation in the signal supplied by the downstream probe during application of the said regulation oscillation is analysed in order to diagnose an operational status of the catalytic converter, the method being characterized in that: - a predetermined number of diagnostic time windows (F2, F2, F3), of the duration equivalent to the period of the said regulation oscillation applied to the regulation setpoint is established; - each time the predetermined number of diagnostic time windows is incremented, a status detection criterion is determined from the variation in the signal supplied by the downstream probe during the said diagnostic time window; - a mean of the status detection criteria across all the diagnostic time windows is calculated, - the said mean is compared against a detection threshold in order to diagnose the operational status of the catalytic converter, and in that maximum amplitude of the signal supplied by the downstream probe over each diagnostic time window is measured in order to determine the status detection criterion over the said diagnostic time window. - a regulation oscillation is applied to the said regulation setpoint and able alternately to impose a rich mixture and a lean mixture upstream of the catalytic converter and the amplitude of which is calibrated so as to keep the value of the mixture within the range of values of the catalytic window during which the efficiency of the catalytic converter is at a maximum; - a variation in the signal supplied by the downstream probe during application of the said regulation oscillation is analysed in order to diagnose an operational status of the catalytic converter, - a predetermined number of diagnostic time windows (F2, F2, F3), of the duration equivalent to the period of the said regulation oscillation applied to the regulation setpoint is established; - each time the predetermined number of diagnostic time windows is incremented, a status detection criterion is determined from the variation in the signal supplied by the downstream probe during the said diagnostic time window; - a mean of the status detection criteria across all the diagnostic time windows is calculated, - the said mean is compared against a detection threshold in order to diagnose the operational status of the catalytic converter,