The advice of replacing the CAT in order to pass a smog check is not recommend, in fact is illegal. A smog check repair station needs to conduct a thorough diagnosis prior to condemning a Catalytic Converter.
It appears in this case the CAT was replaced unnecessarily in 2008, and the real culprit was the air fuel mixture.
Something is causing your BMW to run "hot", creating high NOx. It could be a lean fuel condition, it could be high compression due to carbon build-up on the piston heads; pretty much any condition which will cause your engine to run hotter than normal.
We recommend starting with a wet/dry compression check, followed by a fuel feedback test to ensure your BMW is in proper fuel control, and then a vacuum leaks check, which may be presenting unmetered air to the intake system making the air/fuel mixture too lean.
Above all this however, make sure you are using high octane fuel. Higher octane fuels burn much cooler than low octane.
posted by SmogTips Support 12-02-2011 09:53 AM