can"t pass smog...help!

[dawkid]

93 ranger v6, auto, my truck is running lean and the nox's numbers are too high, I had the cat replaced and there was no change, I replaced air filter, plugs and it still failed, the smog guy told me that I wasted my money replacing the cat and that the problem was a sensor, I went home and pulled the codes and the numbers I got do not exist on the manual, I ndid the test several times and got the same codes, a set of two, the first one is 33 and the second one is 331, I hope somebody knows the fix, I followed the directions on how to pull the codes and I'm sure I'm doing it right.
thanks.

 

[Jason]

If it gives non-existent codes it tends to be a faulty ECM. You could of course partially plug the air filter or dirty up the MAF (if equipped) and richen it up.