If you run cats without an O2 sensor or any type of closed loop computer control they will burn out or plug up in short order. The mixture has to be precise going through the cats to keep them at exactly the right temperature. Too lean and they get too hot and melt, too rich and they plug up and foul. The O2 sensor tells the computer how to set the mixture. If your shop is telling you something else then you need to find a differant one.
