I have read conflicting reports on where to place spacers when gutting only the pre-cat and maintaing the main cat in its stock form. I have nailed down that a CEL can be avoided by the application of o2 spacers applied in pairs on each set of downpipes. However when only gutting precat where do the spacers go, in the pre-cat o2 sensor position or the main cat sensor position? a definitive answer would be much appreciated as my Audi is in pieces in the garage.