An assembly for cleaning blowby gases has an electric heater and a downstream catalyst positioned in series relationship in a blowby conduit leading from an engine's crankcase to the atmosphere. The heater is controlled by a controller to maintain a selected temperature at the inlet to the catalyst as determined by a temperature sensor. The heating of the blowby gases increases the blowby gas temperature to a level where the catalyst is active to oxidize the constituents of the blowby gas stream.