My 1997 BMW 740IL just failed emissions for a sticker. The reading was low threshold 420,430 codes(catytic converters). I had the auto diagnosed by the best local foreign automotive mechanic who is a genius in BMW. He said not to blame the catalitic converter as the culprit until we look at the engine diagnostics. He found the intake manifold gasket leaking and a few old possibly defective vacuum hoses. We replaced the all these components, new spark plugs, oil and filter and acording to the mechanic the engine was absolute maximum efficiency. The mechanic never said this was the absolute cause bur never said it was the absolute cure. As luck would have it the check engine light came on one day later. My question is has anyone placed high quality after market converter(california legal converters) on their car and had any luck. If I have to buy BMW cats, they cost more than the value of the auto. This BMW has 245k miles on it but runs and looks fantastic. I love the car!! Any stories or direction, the cats are the definate problem!!!!!!!