Regarding catalytic converters, I have a friend who is co-proprietor of a 5-bay independent garage, with MoT testing. He tells me that the scrap dealers will give him money for original equipment cats but zero for aftermarket cats, even if they are type approved. Some of the aftermarket cats only last for one MoT, and then just fail on the second. Similarly I have had poor experience with cheap after market lambda sensors and now only fit NTK (NGK) ones, which give a nice waveform on my oscilloscope. If the cat is physically much smaller than the old one, don't buy, and make sure it is 'Type Approved', if required by the Reg. My garage friend says they don't have too many problems with BMW cats, and on my son’s Rover 200Vi the Bosal cat will last a couple of MoTs and is only about £80. You pay your money, and take your choice, as ever.