Bill, the silicone would come from many of the different gasket glues and such. Even if you used sealant on your cold side of your turbo plumbing this can eventually migrate through your exhaust to the sensor. I used to do surface analysis for the electronics industry where silicone would wreak havoc with any kind of plating process and you would be amazed how easily and how well silicon migrates to other areas from the area it was applied. Still my money is on the lead in the gas. John
