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Originally Posted by Brad Roberts
Very interesting!
I safety wire the second set of 02's under the car so they can read "fresh air". Never throws a light. The secondary 02's do not effect the tune of the car and are only looking for cat efficiency (as stated above)
I have done this 20-30 times on the 2.5 race cars running the 5.2 ecu and the 7.2 cars (2.7/3.2)
The later 7.8_40 (997/987) does not like this. Porsche actually dumps a ton of full into the system to see if the secondary 02's exist in the system and throw the CEL if they do need read the predetermined amount of fuel. Porsche does this during normal freeway steady state cruisng 
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Actually I believe the 7.8 DME was first installed in the 03-04 986 and yes it will throw the cel light (ask me how I know) but some of the header have 2 ports so you can install both o2 and they work fine that way.