At this point you've replaced the two components which do control NO emissions but have not been able to pass the smog test yet.
Assuming the repairs were done properly and both the EGR system and CAT system is functioning properly, you want to focus on any condition which may be causing a lean air/fuel mixture. Lean fuel mixtures are responsible for high NOx emissions. NO is created when an engine's combustion chamber temps reach above 2500F. Any fault(s) which allows temperatures to increases above 2500F will result in high NOx emissions.
We'll have to find out if and why your Chevy Blazer might be out of fuel control. The diagnosis should begin a “fuel feedback” test, ensuring the oxygen sensors are working properly, and the ECU is sending the fuel injectors the correct information in terms of increasing or decreasing fuel delivery.
At the end of the feedback test it will be know whether your Blazer is suffering from a Lean fuel mixture problem (which is causing high NOx).
Other NOx faults to concider are: bad engine cooling, carbon buildup in the cylinders due to high mileage, and intake vaccum leaks.
posted by SmogTips Support
