New Headers for my '04 S

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Originally Posted by Stroked & Blown

Is the CEL issue due to the fore (header/manifold) O2 sensor or aft?
The fore 02 controls closed loop fuel trims, the aft sensor tracks catalytic efficiency and has no bearing on fuel trims. If you remove the cat between the two O2 sensors, you can use the non-foulers on the aft sensor without a problem.

Is Porsche using the fore O2 for fuel trims AND precat efficiency?
If so, is this the case for all model years, or just 00-04?

You would get multiple error codes if the primary (fore) and secondary (aft) O2 sensors were not reading exhaust gas. Once the car is warmed up the DME switches from a generic fuel map to a tuned fuel map based on information from the MAF, primary O2 sensors, air temp, engine temp, and driving style. Without primary O2 sensors in and working the DME gets confused and might get the tuning really wrong or just switch back to a generic fuel map which would lower performance and HP. The secondary O2 sensors are for emissions only.

On 2000 - 2004 cars the stock headers have O2 sensors before and after the first set of cats. Ignoring the primary O2 sensors and CEL just doesn't work. The car runs poorly. The trick is to add high flow cats, reprogram the DME to look for new exhaust gas values so it will be able to switch to a tuned fuel map again for max performance and HP. This is best done on a dyno by someone who knows these cars well like Farnbacher Loles. Now the car will run great and pass the Smog sniff test as long as no one looks under the car. Still not smog legal by the book. In California headers are legal on "track only" cars. I don't know of any simple backyard/shadetree/over the counter solution to this issue.

97-99 cars don't have this problem due to one set of cats only. Adding headers is a simple bolt on with no tuning issues. The DME sees normal O2 sensor values and selects a tuned performance fuel map after warmup. Still not legal on street cars in California.