Exhaust gas from an internal combustion is subjected to a secondary treatment in which air is supplied (22) to initiate an exothermic reaction from a secondary air valve (10) via an air outlet opening (38). An additional check valve (32) is provided between the secondary air outlet opening and the exhaust duct (28). Preferably the check valve is heat-resistant and heat repelling. The valve is preferably located directly on a housing (24) in which at least one exhaust gas duct runs, with a duct (26) connecting the outlet opening and the exhaust duct.
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