i have just went through this only with a v 6. i purchased the truck with the deal that the trucks engine light is out. it had the same code as yours does. i got it with the light off and within 75 km the light came back on. the mechanic replaced both the upstream and the downstream and still no fix. then i went to an older friend who gave me the lowdown. when the emissions are not up to specs through the readings from the sensors, the ecu will cut the spark to a selected cylinder every once in a while to dump extra fuel into the cat, or cats in my case, in an effort to get them to light up and burn. what is happening is that the cat, or cats is unable to light up and burn the unburnt hydrocarbons coming out of the motor, and the downstream is sensing that there is no change in the exhaust hydrocarbons. if you plug in your obd 2 and can look at live data, get the truck up to full operating temperature and look at the o2 sensors. im not shure what they are referred to on the live data screen, but the upstream (closest to the motor) sensor resistance will jump around a bit, whereas with the downstream (after the cat or cats) should maintain a constant reading, even as the motor is revved. if the reading is jumping around, the cats have "bit the cookie" and are not burning the unburnt hydrocarbons. after the cats were replaced, it was like a whole different truck. it was more powerful, smoother and quieter, but i did not moniter that much of a change (if any)in fuel economy. good luck!!