It is typically recommend to change the Catalytic Converter only after all other emission systems have been inspected. You must ensure your Nissan Quest's EGR system is working first. This is the most common reason vehicles fail the NO portion of the smog check, but there are other reasons too. The EGR valve is responsible for re-circulating a small amount of (inert gas)exhaust gas back into the combustion chamber. This cools the chamber temperature by producing an inert gas along with the regular fuel mixture. Your Nissan Quest produces Nitrous Oxides (NO) when engine temperature rises above 2500* F. Keeping the chamber cool reduces NOx emission.
You also want to look into fuel control and air/fuel mixture. Components such as the oxygen sensor/s, Mass Air Flow sensor and engine coolant temperature sensor should be inspected for proper operation. Any fault which could cause less fuel to get to the combustion chambers then necessary could and will cause high NO.
For more reasons your Nissan Quest could be producing high NO follow this link.
