There are two different systems; one system is associated with the fuel evap system and the second is associated with the secondary air injection system.
A general description of the fuel evap system is here:
http://www.autoshop101.com/forms/h62.pdf
A general description of the secondary air injection system is here:
Secondary air injection - Wikipedia, the free encyclopedia
The fuel evap canister purge valve is discussed here:
EVAP Canister Valve - 986 Series (Boxster, Boxster S) - RennTech.org Forums
The secondary air injection system valves/reservoir are under the intake plenum and behind the alternator and is discussed here:
P0491 and P0492 secondary air injection codes - 996 Series (Carrera, Carrera 4, Carrera 4S, Targa) - RennTech.org Forums
Hope these point you in the right direction.
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