It is possible I guess, if it is still cleaning the exhaust properly it wouldn't throw the cat converter efficiency faults... But if it is blocked up it usually causes some rich mixture faults and bogging down/sputtering etc. under load or at higher RPMs. The cat converters are expensive from BMW, but I've seen a couple guys use magnaflow universal cats without problems or faults. But I'd make really sure you actually need cats before you worry about that as the symptoms don't match up.

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2003 4.6is