Had trouble passing my 1995 2.5 YJ, high NOX always seems to be the problem.
At 200k I had replaced the Cat thinking it was time. It passed but two years later I had the same problem.
The mechanic at the station explained to me that the Jeep was running too lean to fire up the Cat and that aftermarket Cats are notorious for it. He finally got it to pass and I asked how he did it.
He told me that he pulled a spark plug wire while it was running, letting unburnt fuel to reach the Cat,
then plugged it back in still running. He rechecked again and it cut the NOX in half.
