Ok, I just had smog done on my 96 Civic DX. It failed the NOx. Any suggestions to fix this?

I did see about possibly replacing cat & adjust timing. A cat is a bit expensive but if I have to do that, then I will.

 

When was the last time you replaced your O2 sensor(s)? When I replaced mine, NOx dropped 60%.

 

Did you get the car "Hot" before the test?
Meaning did you drive it for around 30 minutes before you took the test?
You need to get the cat hot for it to work correctly.

If you do a search you'll find many threads about what to do and what not to do, to have your car ready for a smog test.

 

The cheapest and easiest thing to try first would be some injector cleaner - anything that will remove carbon build-up on the exhaust valves (assuming there is any substantial amount) can make a big difference.

Next, before buying any new parts, I'd check the ignition timing as you mentioned.