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This issue has been the cause of much controversy over the years. Please note that we as a club can only recommend a course of action. It is up to you, the individual if you would like to take the advice or ignore it.
The E51 has what is known as a Quad-Cat system. This means that there are 4 catalytic converters in total. Two for Bank 1 of exhaust manifold of the V6 engine, and then another two for Bank 2.
The issue from this system is that over time the first catalytic converters from each bank can deteriorate over time and can break down. The debris from the broken ceramic material then created a blocked in the secondary catalytic converters further down the exhaust. Once a blockage has occurred, pressure builds up, exhausts gasses and debris can back flow into the engine and create terminal damage to the engine.
The best solution to this problem is to remove ALL FOUR catalytic converters and replace them with TWO high flow sports cats. Now this can be a very expensive procedure but gives 100% safety.