if your cat is broken you will have to replace it cause the noise will make you crazy.some cats that are broken will still work enough to pass another smog check. the piece inside may be in two large chunks or may be small enough to get inside the resanator or muffler. this leaves you with more rattles.
when the mixture is correct what is inside your cat will clean up and you will get your best numbers possible.
i have seen a lot of broken cats pass the test. but what about the chunks inside. do you really want them in your muffler?
fix the mixture problem,drive a hundred miles and retest. this will tell you what to do next. good luck
food for thought and maybe a different tread. this timing resistor that is removed for power, what if a resistor that retarded your timing more, instead of less was made and put in before a smog check. that should improve all the tailpipe numbers.
this would be for cars that almost pass smog.
nox,co and hc would go down. will experament with this soon. tail pipe numbers with and without the resistor.
got carried away again 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