Looking at the numbers from your test, it looks like you are running just a hair rich and your cat is getting old. Most closed loop emission systems will try to run perfectly centered to slightly rich. If the engine runs slightly lean, NOX readings climb rapidly due to higher combustion temps. HC reading go up with a much gentler slope on the rich side and they figure the cat can clean them up. Harder to clean up NOX. Trouble is, as the cat gets old its efficiency starts to fall off. That's why all the comments about keeping it real hot before the test are correct. Keeping it hot like that keeps it operating at whatever its current peak is. 30k miles on an oxy sensor is enough for its response to fall off or for it to become a bit contaiminated. Changing it probably won't make a big difference in the numbers, but might be enough to pass. Those things commonly go for 100k. Personally, I'd find out how much you have to spend to get a waiver if it still fails, and then make sure you reach that number in counted repairs. In my area you have to spend $200 to get around the test. Oxy sensor, plugs and wires will put you there pretty easy and it's all stuff your car needs form time to time anyway. I'd probably adjust the system to run just a touch leaner since they don't seem to care about NOX.