California requires OE cats, no aftermarket. The Volvo programs are very tight and it doesn't take much to trigger a fault, the after market cats don't last very long and will generate a code in a couple of years.

BUT, not all Volvo cats cost $1200:

http://stores.revolutionparts.com/tascaparts.com/volvo/s70/9189238/1998-year/glt-trim/2-4l-l5-gas-engine/exhaust-system-cat/exhaust-components-scat/?part_name=converter-and-pipe

Before you spend a bunch of money, you still need to find out what is causing the cat to die. Cats can last forever, not just 10 years. If the mixture is too rich, then it is probably unmetered air getting into the intake manifold. Other reasons for rich mixture include the fuel pressure regulator, MAF, MAP.

When testing the O2 sensors, the rear sensor should have a constant reading and the front should fluctuate as it alters the fuel/air mix.
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My back feels better when I sit in a Volvo seat