Well, all I know is, the tech who ran the test (who I really trust) had me do every possible improvement, adjustment, tune-up, etc before replacing the cat, and it didn't pass. Installed a new cat and it did. It didn't look that bad, looking through it, but, it wasn't doing a good enough job.
So, make sure all plugs are clean, gapped right, and firing with no missing. Clean up those distributor cap towers (internal and external) and rotor. Check the timing. Look for leaks in the exhaust as well as the intake. Change your oil just prior to testing, and make it a heavy straight weight to reduce burning. It all worked for me.
Good luck.