My car stinks, literally. I am on the 4th o2 sensor inside of 12 months which also included replacing the cat. under the 80k emissions warranty.

Have you put "leaded" gas in your car lately? If not, the cat should last 150 - 200K miles. Your "dealer" might not have installed the O2 sensors properly if that initially turned the CEL off. Go to an Autozone and get the OBD codes and post them here. We can help better that way.
John is right, find another mechanic before you spend enough money to send his kids to med school.

Klaus
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98 V70Rawd(101Kmi), 95 854T(85K mi), 88 245(165K mi)

Run away from that dealer and don't patronize them again. Dealers are in the business of replacing expensive parts to make money, they hope that will clear the problem and if it doesn't then replace more parts.

Find yourself a reputable indie shop that can work on volvos, better even learn how to do some minor maintenace and repairs yourself and save yourself a ton of money. In the meantime invest $99 in your car and get yourself a good OBDII code reader and read the codes yourself. Vaccuum leaks are common in these cars and trigger the CEL light on. It's a cheap fix, like replacing a $3.00 rubber elbow but it takes patience to find them.
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'88 240, '92 745, '98 v70 John, Tampa Bay