I have a 2007 4WD Tahoe LTZ with the 5.3 Flex Fuel engine. Since January I have had to replace the Catalytic convertors 3 times. The first time was in January at 175,000 the second time was at 179,000 miles, and now the third time at 195,000 miles. There is a slight, but seemingly insignificant exhaust leak from the exhaust manifold to the head, can't hear it with the windows up. The only time it is noticeable is when next to a building with windows down, or when outside the truck while it is running... What could be killing the cats? Any ideas? I have read anything from an intake leak, a EGR valve, to a blown head gasket or something similar allowing coolant to get into the exhaust. The o2 sensors were replaced in January with the first Cat system. (They replaced the entire Y setup). It idles so well that sometimes I do not know if it is running.... Other times there is a slight shimmy but almost unnoticeable... I use quality gas, Mobil, BP, Shell etc... Every now and then a cheap gas, but usually I get better MPG.... This last time I had been running Ethanol Free Premium for about 4 fill-ups when out in Yellowstone. I have not run E85 for about 4 years... Anyone else have anything like this happen?