help on question regarding O2 sensors on 06+ Civic Si

[soxfan143]

I need help badly. I installed a Greddy Turbo on my Si and I was having some trouble with the tuning. I sent it to a local shop and it was rewired and tuned. My problem is they are telling me that my fuel trims are going crazy because I don't have a Cat in there. They are telling me that the secondary O2 sensor is reading that there is no cat and it's sending my fuel trims off and causing the primary O2 sensor to fight it. This is in turn causing irregular fueling and they can't stabilize it. So now they are telling me that I have to install the cat back on and have it retuned again. If anyone can help me before I pay them to do all this for no reason. They've had my car for 4 days now and this is the answer I got. This makes no sense to me at all. Please help me.