Problematic Lantray! need help with emissions

Hi guys i need help, my lantra's failed its MOT on emissions.
 
Its a 2L CDX 98 model 80k miles and gets 9.99 on fast idle with the CO, hydrocarbon count is iratic.(normal idle averages 7.00 and HC about 300'ish)
My mate owns a breakers yard so i've tried 2X2L cat's and a 2L coupe cat(yea the manifold fits fine its less prown to cracking too and detachable from the cat :\).
Anyway tried treatments, theres no visable blue smoke for burning oil,no problems with idling speeds, air filters ok, good petrol in it, it is getting to a good running temprature and o2 sensor reads ok. anyone got any experiance with this sort of problems? much appreciated. electronic diag. isn't my strong point more of a nuckle head :\.
Also where's the 16-pin diagnostic socket? :S
Cheers
 

I'm not much help either.  On American Elantras from that year, (left-hand drive cars, no diesels sold in U.S.- not sure where you are), the connector is located near the hood release (labeled I03):



And schematics for the connector.