Any smog experts out there?

MOPAROFFICIAL said

You say you have no codes like a p0441. Does it have evap? Is it four-wheel drive and could there be a crack in a vacuum line that goes down around the transmission to some hard lines into the front axle. EGR stuck open not closing all the way?
It would be a bummer if a crack on the exhaust seat made it to coolant that'll cause a misfire, otherwise it could be cracked into the intake seat as long as it doesn't have coolant leaking through keep running and not bring any cells

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No codes of any kind. It runs great....no misses (which should show in the HC and CO readings too) 5 speed manual 2 wheel drive. Coolant is clean and full, no overheating problems at all. We could probably throw in a brand new cat and get it to pass. I'd prefer to figure out exactly what' making it fail instead. We'd also like to keep the price down on this beater too. I plan on starting with decarbonizing the combustion chamber. That's cheap and easy to do. Maybe put in a new O2 sensor too since I think it's the original (and 150K on the odometer). Then the free retest to see if that does the trick. Thanks for all the ideas everyone.