Results with a modified dual catalyst system incorporating Gould's Nitrogen Oxide (NOx) reduction catalysts are presented. The key factors controlling durability are examined through laboratory, engine dynamometer, and vehicle tests. Test data is presented showing that control of oxygen input to the NOx catalyst can result in significantly improved durability. Results indicate that by combining oxygen input control with a new low ammonia forming catalyst, stable NOx catalyst performance can be achieved through 25,000 miles of AMA mileage accumulation. Potential failure mode factors are also discussed.