There are several reasons your Datsun 300ZX could encounter NOx failures. The most common is the malfunctioning EGR system. Other problems may include high compression, high engine temperatures, and/or vacuum leaks.
Nitrous Oxides emissions are formed when engine chamber temperatures rise above 2500* F. Keeping the chamber cool reduces NOx emission.
The fact that your smog check VIR (vehicle inspection report) indicates the CAT and vacuum lines "passed" only means that the smog technician visually inspected the these components/systems for proper connection. "Pass" does not indicate that the components are functioning properly. We'd recommend first inspecting the EGR system for proper operation, and then looking into the fuel injection system for proper fuel control. Also remember to inspect for vacuum leaks.
