Do you know the the previous owner took good care of his car before destroying the engine? Sometimes cats get destroyed because people will not replace spark plugs when they are supposed to, and sometimes when someone neglects oil changes, oil consumption goes up, and both of those situations are bad for cats. Other times, the cat fails first, bits of the cat get sucked back in the engine, destroy piston rings and cylinder bores, causing a downward cycle. This was common on the Nissan Altima 2.5L.