I have a Nissan Stanza 1991. It passed Emission tests since I got it in
2001. But this year it failed in all the three tests NOx, HC, and CO.
The car has 197K+ on it.
I change the O2 more than a year ago (I got the BOSCH universal O2 sensor on it). Oil was due to be changed by the time of the test but I didn't change it. I have never changed the CAT on it. By the way, does the CAT get fixed or they just need to be replaced? (Excuse my ignorance).
I noticed before sometime before the test some blue smog coming out of the exhaust when I start it but it goes away in a little while.
I had put in the fuel tank a bottle of some Smog Reduction additives. It was already in the tank the car test with.