I don't get it. Aren't there 4 sets of O2 sensors on each sets of cats? Right. The 2 on the 1st set which are located on the headers and the 2nd set which are on the down-pipe just before the exhaust. So that makes a total of 4! As long as you keep the 1st set with the headers that use a bung hole for the O2 sensors the cell should not come on. The second set of cats also uses O2 sensors, and if you were to buy an after market set of by-pass pipes they would still need the bung holes for the sensors. So maybe the Porsche master tech thought the removal meant the sensors too, and not just the cats.
