I checked codes the other night and had a code for a bad rear O2 sensor. Through research, I discovered that the bulb had been removed for my check engine light. Installed the bulb this afternoon and took the truck for a run. Borrowed at remote thermometer from a local mechanic and checked the temperatures at the front and rear of the cat as SteveRam suggested. The temperature at the front of the cat was about 80 degrees hotter than at the rear. (480 degrees front, 400 degrees rear, truck running, fully warmed up.)

The cat also has a welded seam running across its width, in the middle. I'm guessing the cat was punched out, but wanted some verification from others. From what I've researched, a plugged cat will be hotter in the front, but I'm not sure how much different the temperature would be if it was plugged. Also, the truck runs great, none of the symptoms that others have described from a plugged cat.

The next question concerns installing an O2 simulator. I already ordered one from O2simulators.com and wanted some input from others who have used them. From what I read, you leave the O2 sensor intact, cut only the signal wire, and splice the hot and ground wires from the simulator into the harness for the sensor. How about the heater wire for the O2 sensor? Does that stay intact? I guess this way the O2 sensor is still functioning, but it isn't sending any signals to the PCM. I'm just curious why you wouldn't cut all the wires to the O2 sensor and seal them off.

Picture of cat added.

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