Liz from Alexandria called in with a 2002 Subaru Outback that she wanted to sell, but the check engine light came on. The shop found the code was for catalyst efficiency and sold her an aftermarket cat. Two days latter the light came back on with the same code. You advised her that she needs to have the shop upgrade the cat to a factory type cat for the difference in price. If she does this, she will have spent the money and still have the same problem. A cat wont work right unless the car is in full fuel control. You have to ask yourself, “Why is the cat not working?” and then address that. Once that is addressed and the car is in full fuel control, give the cat a chance to work. If it doesn’t, then replace it. Shops around me replace five cats a week. We replace one every month or two. I suspect her front o2 sensors are slow or lazy. 250ms or longer switching time will cause a cat to go to sleep.