To produce and market vehicles around the world, manufacturers must meet the current pollutant emission standards. CONAMA - CONSELHO NACIONAL DO MEIO AMBIENTE - is the creator of PROCONVE - PROGRAMA DE CONTROLE DA POLUIÇÃO DO AR POR VEÍCULOS AUTOMOTORES - entity that governs the amount of emission of pollutant gases admissible for a vehicle to be homologated and later sold in Brazil. This article is based on the most current version of PROCONVE, which is P7, according to RESOLUÇÃO CONAMA no. 403, November 11, 2008, published in DOU n° 220, November 12, 2008, Section 1, page 92 for heavy vehicles, with a maximum total mass exceeding 3,856 kg or mass of the vehicle in running order of more than 2,720 kg (heavy passenger and / or cargo vehicles). It seeks to convey to the reader the knowledge about the operation of the SCR - Selective Catalytic Reduction system, one of the models adopted by diesel-powered heavy-duty vehicle manufacturers to reduce the NOx (nitrogen oxides) emissions, so that vehicles can meet the standard P7 and be marketed in the country.