Hi guys i need help, my lantra's failed its MOT on emissions.

Itsa 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 themanifold fits fine its less prown to cracking too and detachable from the cat:\).
Anyway tried treatments, theresno 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 ando2 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.