Quote Originally Posted by HWORRELL
He is failing on NOx, 02 sensors will not cause a NOx failure,unless it's very very lean but if it was it would usually cause an HC failure also. 4 things will cause a NOx failure,
"in order of most common"
1....EGR system
2.... Catalytic converter.. usually will fail NOx along with HC.
3...Advanced ignition timing..
4.... overheating engine...
I changed the EGR valve on the truck ,and passed with flying colors!Thanks for the advice. Also my truck has one of those special EGR that can't be checked like the normal ones