I need some help from the experts on this site. I had a different thread on the same topic but it wasn't really going anywhere. So maybe this time will change things. My problem is a CEL with the codes being P1133 and P1153. Each time I take the truck in and have it tested everything passes. Yet I drive to work and when coming home at night, in the same spot every time, the light comes back on. I don't notice any problem with acceleration, start up, or miss fire at all. I've been going round and round with this and the light never stays off more than a day. I've replaced the cats, all 4 o2 sensors (both #1's twice), both manifolds, and manifold gaskets, and had the broken manifold bolts all replaced. I had everything but the cats done at a Chevy dealer and they have told me that if the most resent o2 sensor swap doesn't fix the problem they don't know where to go next. Do you guys have any suggestions? I live in an emission testing county so I need this light to go away or the truck will have to go. Please help!!!