Amended thread for 95 catalytic converter problem
 

Thank you to those who offered help but I now will restate the problem.

Purchased 95 honda accord and couldn't get it smogged.( Ca.) No check eng or any lights came on to indicate a problem. Failed tailpipe test indicating nox level too high @ 25 mph, 700-800 ppm. Then garage diagnosis said to replace O2 sensor and cat. Am now running premium and did injector additive hoping to pass, no luck first try.( going to completely empty the tank this time) I don't want to replace cat unless I'm absolutely sure it has to be replaced. Any ideas that could help to pass? Thanks to all on the forum. ( 95 model only has one sensor and if I can't get it to pass I'll replace the O2 sensor first and try again before replacing the cat)

How about your HC levels?

NOx and HC can be played off against each other (to some extent). Retard your spark timing a bit. That would reduce NOx but increase HC. *IF* this is a 4-cyl you'll twist your distributor clockwise just a bit (standing at the right-front wheel looking at the distributor).

There's a sweet-spot where both HC & NOx are moderately lower. That spot is supposed to be centered in the spec range of spark timing (right on the biggest of 3 notches if you use a timing light).

EGR should be kicking in @ 25mph. Are the ports clean?

Clean the EGR ports. See the common diy thread on top of the gen tech help from for a link on how to do this on 94-97 accords.

x3 on the ports :)