My emmisions were high and I switched the Cat. and all was good. They just visually check the Cat. The smog tech. cant really tell if it is working properly or not. It will just give a bad smog reading like high HC. After My car barely passed I ended up hearing the Cat rattle around soon after. I tuned the car with the bad cat as best as possible then got a new one.
MY readings are now pretty much around 0% CO and 7ppm HC. Before the new Cat. 0.5% CO and 170ppm HC.

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Chris
84 280sl
82 300d euro