An assembly and method for reducing nitrogen oxides, carbon monoxide and hydrocarbons in exhausts of internal combustion engines, wherein the exhaust is acted upon in a first stage catalytic converter. A first portion of the first stage catalytic converter output is cooled and a second portion of the catalytic converter output is not cooled. The cooled and not cooled exhausts are united and directed to a second stage catalytic converter. Air is injected into a selected one of (1) the not cooled exhaust prior to the juncture thereof with the cooled exhaust, and (2) the combined cooled and not cooled exhausts after the juncture thereof.