Passed emissions, NOx just barely

I'm in California, got a 91 740 wagon with Regina engine, 260K miles.

I just passed the smog test, but only barely for the NO at 15mph. Max
is 785, I got 781. When he revved it up for the 25mph test, NO dropped
to 518. I had just driven the car for 25 minutes or more on the freeway
before the test.

I got a new cat converter 4 years ago. He said they usually last about
that long. That doesn't seem right.

So, how long do cat converters last, on average?

 

New ones (manufacturer) are required to last 100,000mi. 4 Years sounds
like they're trying to sell you one. They can become less effective if
the engine is burning oil or dumps unburnt fuel into the exhaust. The
whole point of a catalyst is that it's not used up in the reaction.

Bill

 

Mine had been leaking oil for quite a while, but not burning out the
tailpipe. The flame guard got plugged and blew some seals recently.
Got all that fixed. I wonder if all the times I ran low on oil created
fumes that got to the cat converter.

That oil leak problem is fixed now, thankfully, now that I've found a
decent mechanic.

Will my recent slower driving improve or hurt my emissions?