some of these aftermarket cats are just plain junk. catco is one i can think of in so. cal. there is nothing like a factory cat for most cars. a lot of cats are replaced because fuel mixture was wrong,too rich, which shut down the cat. fix the mixture problem ,take tailpipe readings and the cat is not working.drive the car 20-100 miles and the cat will come back to life. readings are like a new car. this does not happen much because customer wants instant results, not maybe it will come back.
also they wait till the last minute. smog due yesterday. as soon as an aftermarket cat goes on a car ,there is a good chance it will need a new one next smog check. the nox part of these cats are weak.at least half the time the cat was put on with out fixing the fuel mix problem,so again new cat next time.
sorry bout this long reply. the other responses where also right on. try to save the factory cat.
while dealing with cats, a lot of techs are not aware that obd2 fuel mix MUST go rich to lean at a fast pace, if biased rich or lean the nox part of the cat does not work very well. the mix has to cross over the stoich point for the cat to save then use oxygen for nox reduction. biased o2 sensors make the cat look bad. got carried away here thanks rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car