Well, I finally passed. Here's a little summary that might be useful to someone or not.

Bought a 93 944 with 170K, with safety but failed emissions test (failed on HC, CO and NOx, passed at idle) that really wasn't running badly and looked well maintained.

It had a one year old aftermarket cat (whole exhaust from the y pipe to the tailpipe looked new). Having never replaced a cat in 30 years of driving I cross this off the list of possible problems. This was a mistake.

What I think happened is that the car was mildly misfiring due to a bad ground at the coil and the previous owner replaced the cat to pass the E test, then drove the car for a year or so and ruined his new cat.

I got a little lost hunting down the misfire because it seemed to idle too well to be electrically related and I chased down a non-existent or minimal vacuum leak until I was blue in the face (I can remove an intake manifold blindfolded in less than 10 minutes now however).

I have gained a new respect for the importance of good grounds.

Ron
93 245
93 944

near Brantford, ON, Canada