If it was within the mileage/time limits, the converter warranty would only apply if the converter failed for certain reasons, ie manufacturing defect, etc. It would also likely be covered under the emissions warranty if it was deemed to be consequential damage from the failure of a covered component, ie the oxygen sensor.

If it failed for another reason, for example overuse of fuel additives, warranty would not apply in any case.

This does raise the question of why it failed. If there is no obvious cause (such as continuous use of non-recommended fuel additives) I would have the dealer check out the other systems to make sure the replacement converters did not fail similarly.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue