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California is the only state that requires OEM cat converters. While 'aftermarket' Cats are not as good as OEM, they still work quite well and will last for years.
The most effective part of the system is the O2 sensors. Due to Volvo's stricter than required emmissions policy, the O2 sensors need to react very quickly and some of the aftermarket do not, or they loose capacity in 10-20K miles.
Check the exhaust system for leaks before the cat also.
No need to replace the vacuum 'tree' unless it is cracked and/or leaking.
Your engine has a hidden vacuum line close to the #1 cyl intake. It is notorious for cracking due to heat. Very hard to see, so just feel with fingers, any roughness will probably determine if it need replacement.
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My back feels better when I sit in a Volvo seat
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