I have a 98 Dodge ram 1500 5.2L 318 with 153,000 miles. I have failed emissions testing three times. Did tune-up, replacing plugs, wires, rotor, and distributor cap. Ran fuel system cleaners. The third test showed significant improvement for HC, and CO, both passed. However my truck is failing the NOx test. I took it into a shop to have it diagnosed, the technician said everything checked out good, including the O2 sensors. He told me for $250.00 he would do a chemical treament to clean valves and injectors, and it would take him about four hours to do. He believes this would solve the NOx problem. Also I was told my truck does not have an EGR valve. Any suggestion on how to lower the NOx emissions without replacing the catalytic converter (which was replaced with an after market cat about 12,000 miles ago)?