My 1993 240 (145k, no EGR i think) just failed its NYC emissions test! The HC was ok 0.50 (max 0.80) and the CO was ok 5.95 (15.00 max), but the NOX failed 3.35 (max 2.00). I put an aftermarket Cat on it only two years ago but didn't buy the new header pipe to match the modern Cat's flat flange (i was in a hurry and found that the old header with its donut flange didn't match when i got under the car in my school's parking lot, so i jury-rigged the gasket). So i may have an exhaust leak at the coupling adding to my high NOx as was beautifully described in a recent thread here.
The mechanic who tested the car said don't buy an aftermarket cat, they barely last a year. He wants to put a Volvo OEM on for me with a new header pipe to met the new flat flange of the Cat. The Cat price is roughly $700! So, i'm thinking of buying an aftermarket Cat for less than $200 and a Starla OEM pipe for about $150 and do it myself. I am worrying about the quality of the aftermarket Cats such as Bosal, DEC, etc compared to a Volvo part. I'm also worrying about breaking the studs at the headerpipe/manifold joint, but that's another thread.
Thanks.