CVO 1/31/2013NOx (oxides of nitrogen) is a tailpipe emission caused by the oxidation of nitrogen in the combustion chamber. When the peak combustion temperatures go over 2500°F (1371°C) NOx is formed in excessive amounts. To keep the combustion temperatures down, exhaust gas is recirculated.
Re-circulation of the exhaust gases is accomplished by having a movable valve between the exhaust and intake manifolds. Upon a predetermined demand, engine vacuum is routed to the valve, opening the connecting port and allowing exhaust gases to enter the intake tract.
Check the egr valve.