Catalytic Warranty Replacement, o2 sensors, heat shields

My 2004 Mini Cooper with 78,000 miles has a federally required warranty on catalytic converter to 8yrs or 80,000 miles. The authorized dealer found that the converter is defective because it rattles under vibration but replacement wasn't authorized by MiniUSA because they said it still works. A person at Environmental Protection Agency said that Mini would never replace my converter unless there is an OBD error indicating bad converter. Is the bad converter code given based on the readings from only two O2 sensors? What is that code readout?

Answer
    If your "check engine" light is not illuminated, then there are no codes to be read.  The OBDII cars have an oxygen sensor both in front of and behind the converter; the computer compares the readings from the two sensors to determine whether or not the converter is still doing its job.  The EPA mandated warranty only requires that the converter do its job of scrubbing pollutants, not that it be quiet.  It is not uncommon for one of the heat shields on the converter, either an internal one or an external one (there is an extra layer on the top between the converter and the floor), to become loose and start to rattle.  All of this stuff is made from stainless steel, and welding stainless, even in a mass-production setting, is difficult to do well.  One of the welds probably broke, and that caused the shield to bust loose and begin to rattle.  If you can identify the heat shield that is rattling, and if it should somehow become detached (wink wink, nudge nudge), then there would be a safety issue due to the extreme heat of the converter and they would have to replace it.  If the rattle really bothers you, try contacting Mini yourself, or BMW, or whoever.  If you bitch and moan and threaten loud enough (a letter on a lawyers letterhead works wonders), they will authorize the repair.  It would probably be worth investing a little time and money in this endeavor, 'cause Mini converters are quite expensive (which is why the dealer doesn't want to fool with it-he makes no profit on warranty repairs, but will make a boat payment when you come back after the warranty expires).

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