Since you already replaced your Chevy Venture's oxygen sensor/s I'm going to
assume the "fuel feedback" system is operating properly and fuel delivery to
the combustion chambers is adequate. If your Venture was not sending enough
fuel to the engine you'd have a lean fuel condition and this would cause
high NO. So that aside, you want to inspect the EGR (exhaust gas
recirculation) system, specifically the EGR valve. The EGR valve is
responsible for recirculating a small amount of exhaust gas back into the
combustion chamber. This is done to cool down the chamber temperature by
producing an inert gas along with the regular fuel mixture. Nitrous Oxides
emissions are formed when engine chamber temperatures rise above 2500* F.
Keeping the chamber cool reduces NOx emission.
Your Chevy's EGR valve should be inspected to ensure its working properly.
The EGR valve should be able to open and close without sticking and have its
passage unrestricted to allow ample exhaust flow to the combustion chambers
as needed.
Other reasons your Chevy Venture may have a high NO problem are bad engine
cooling, lean fuel condition (which we covered through the oxygen sensor;
though the entire fuel feedback system should be diagnosed) and carbon
buildup on the piston heads and interior of the combustion chambers due high
mileage. These faults will cause your Chevy Venture to fail for high NO
also.
