A procedure for controlling an exhaust gas purification system of a combustion engine, consisting of one or several catalytic converters for converting and/or storing at least one exhaust gas component of an exhaust gas of the combustion engine at least one of the catalytic converters is construed as oxidization catalytic converter. With the aid of the adsorption capability of at least partially uncombusted hydrocarbons of one of the catalytic converters a diagnosing forecast about the NO oxidization of the catalytic converters is derived, whereby the at least partially uncombusted hydrocarbons are introduced into the exhaust gas duct at least partially in front of the catalytic converter. With the procedure and the device for implementing the procedure the NO oxidization capability of the involved catalytic converters can be evaluated in the range of extended on-board diagnosis requirements to provide “feed-gas” for subsequent exhaust gas after-treatment systems.