I just replaced the catalytic converters on my 2002 Chevrolet Suburban with the 5.3L Flex Fuel engine ("Z" engine). I installed a Walker 50511 catalytic converter assembly. It went on very easily and fit perfectly. I also replaced all four of the oxygen sensors with OEM Denso sensors. I put the vehicle on a logging scanner and scanned all four oxygen sensors. The upstream sensors switched back and fourth between the normal extreems with the expected rapidity. The downstream oxygen sensors stayed mainly at one extreme. One was at .8 volts and the other around 0 volts. The downstream oxygen sensors' voltages would change when a dramatic event occurred, such as hard acceleration, but would eventually go to one of the extremes. No codes are pending and the Check Engine light is not lit. I only have about 20 miles on the new converters. Should it take some time for the new catalytic convers and oxygen sensors to "break-in" ?