in may i went to a chevy dealer where my buddy worked and had him look and see why my truck doesnt run all that well. he plugged intheir diagnostic tool thing and it said that my motor was running really lean, so it said to replace the O2 sensor (i think it was running at 400 and was supposed to be running at around 700...im not sure on the units). So i replaced that sensor and nothing changed. Does this mean I have a problem with my ECU? my truck idles very rough (like its about to die) in cold weather and it improves a little bit in warmer weather, but it still runs terrible. any help would be greatly appreciated. Thanks, Dylan