Not offhand...
High NOx can be caused by a lean AFR (elevating combustion chamber temperatures, and oxidising the nitrogen in the air) or by elevated combustion chamber temperatures (oxidising the nitrogen in the air.)
A lean AFR can be caused by a clogged injector nozzle - either totally or partially, but you'd be more likely to notice if it was totally blocked - or by an intake manifold leak somewhere (check for a vacuum leak.)
NOx is typically controlled by two systems -
EGR/Exhaust Gas Recirculation. This is a "controlled leak" of exhaust gas into the intake stream, which is intended to displace oxygen slightly. This enriches the mixture, because our fuel injection systems read pressure and temperature to meter fuel - but don't do anything to note composition of intake gasses. The EGR can stick open, which will have an effect on emissions, or it can stick closed - which will have an effect on emissions. Also, the EGR base gasket can "burn through," which will cause a vacuum leak.
Redox catalytic convters. Our catalytic converters are actually two-stage devices. The first stange is meant to catalyse the oxidation of CO into CO2, while the second stage "reduces" NOx into N2 and O2. A failing catalytic converter can pass more NOx than normal.
While the manufacturers can say what they will, I don't think I've often heard of a catalytic converter lasting more than 10 years - 15 at the outside. They figure most people won't keep a vehicle that long... So, I'd change the cat, check for a vacuum leak at the base of the EGR, and maybe pull the valve and change the gasket/clean the valve (don't be surprised if the gasket and/or valve are OEMR only.) The EGR can be cleaned with any decent commercial solvent - carburettor cleaner is good - and an old toothbrush. When you replace the gasket, a light coat of RTV copper on both sides will go a LONG way toward an effective seal. Make sure you clean both surfaces before you reassemble.
You might also want to check the EGR valve itself for a leak - you'll need a hand vacuum pump. The engine does not need to be running for this.
1) Disconnect the vacuum line from the EGR valve.
2) Connect the vacuum pump line to the EGR valve
3) Operate the pump to pull 15-18" Hg vacuum on the valve diaphragm. Watch the valve "pintle" and make sure it moves.
4) Watch the gage for two full minutes. You should not lose more than 1" Hg of vacuum. If you do, replace the valce, because the operating diaphragm is leaking.
5) Disconnect the vacuum pump, reconnect the vacuum line.
5-90