check engine light 97 legacy GT

I have been having continous problems with the check engine light comign back on. I have brought it to subaru dealer 3 times with very costly bills. The light is back on and I brought it to auto zone and they are telling me that it is catalytic deficiency under threshold, ANY idea what this means, just curious before I go back to dealership again for another repair.

LL

Chances are its the front O2 sensor. Theyre usually the culprit. I dont see why you would have 'costly bills', the subaru dealership should know exactly whats wrong, unless theyre complete boobs