An apparatus for reactivating a sulfur poisoned oxidation catalyst operating in the exhaust of a lean burn, methane source (as in natural gas) fueled combustion device as in an engine. The reactivation includes desulfation of the poisoned catalyst through the use of a CO supplementation apparatus in communication with the control unit that is adapted to supplement the CO content in the exhaust reaching the catalyst, while avoiding an overall rich exhaust atmosphere at the catalyst. An example includes the added supply of hydrocarbons to one or more, preferably less than all, of the lean burn engine's combustion chambers such as by an ECU controlled extra supply of NG (e.g., CNG) to some of the combustion chambers. Also featured is a method for desulfation of an oxidation catalyst of a lean burn CNG engine by supplying excess CO to the exhaust reaching the catalyst while retaining an overall lean state, and a method of assembling an apparatus for reactivating a sulfur deactivated lean burn NO engine catalyst by assembling a CO supplementation apparatus with a control unit.