1993 Audi 100 fails smog NOx test for California

The audio 100 v6 had 203 k but two years ago I had both caytalitc converters replaced and the NOx levels where both around 49 way below the 700 which is the high level in CA.
Now two years later everything else in the test was exactly the same but the NOx levels are now above the max level around 750. What could be the problem?
I have heard the things to do is replace the air cleaner, get some of that guarntee to pass gas additive etc. I have replace the plugs and new wires. It uses a magneto so there is no distributor to mess with.

Not sure about NOx sensors.

Any help would be greatly appreciated.

Thanks