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from the great amount of knowledge that i have amassed from the internet, this is what i think i know:
porsche boxster (other than early 2.5l) has two sets of cats - pre-cats located on the headers, and secondary cats located on the mid pipes.
boxster has two sets of 02 sensors: one set on the headers before any cats to determine efficacy of fuel/air mix, and a second set after the pre- ats on the headers to ensure that the cats are working.
if you put headers on that do not have cats you have four options:
1) live with the cel you get from the second set of 02 sensors telling the computer that the cats are not working.
2) extend the second set of 02 sensors (two wires each) to a location after the secondary cats on the mid pipes. this way the computer will still know you have cats in the exhaust system.
3) fool the o2 sensors. a quick seach will provide several options. if in the us of a you can buy 'foolers' (maxspeed motorsport sell a set, as should most entry-level autoparts stores). these are devices that distance the o2 sensors from the direct exhaust flow. the sensors see less exhaust as a result and think that the cats are there and working. can't find 'foolers'? go to an autoparts store and get a couple of sets of 'spark plug savers' ($6/set) - devices designed to distance the spark plug from the combustion chamber to prevent fouling - same thread pattern as the o2 sensors and bungs; drill them out and use them to distance the o2 sensors from the exhaust flow.
4) get a row flash. all piwis devises come with the ability to flash either row or us. you can buy one yourself ($4k) or find a local mechanic who has one and get it done. benefits are (a) start car without depressing the clutch, (b) computer no longer monitors secondary air pump operation, and (c) computer no longer monitors cat effectiveness (ie, no cel with catless headers).
no rockets, no science. we are smarter than the computers in our cars.
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