Mikle48

I have a 2001 Subaru Outback with approximately 120,000 miles on it. I am not much of a mechanic so I apologize in advance if I appear somewhat befuddled. A little while ago my Outback's "Check Engine Light" came on so I took it to the Subaru Dealer to diagnose the problem. They said they found a, code for catalyst outside the efficiency threshold. I think the code is P0420. They cleared the code and put a can of BG 44k in the fuel and said this will sometimes help, and it did keep the check engine light off for most of the first tank, but then the check engine light came back on again. I put another can of BG 44K in the second tank, but it didn't help. The third tank I again put BG 44k in, but this time I used premium gas. Soon the light went out which made me happy, but when the tank got down 3/4, the light came back on again so I have aborted using the BG 44k (at $25 a can it is pretty costly). In all fairness to the deal, they did say that the use of BG 44K was a long shot, but the alternative is a costly $1800 repair, which I assume is a new catalytic converter. So I am hoping to get some thoughts, ideas, comments on whether to proceed with the repairs or not. It makes me wonder what caused the problem in the first place and will it happen again to a new converter. I would like to keep this vehicle on the road and don't want to cause permanent damage, but the estimate makes me wonder if it is a good investment. Your ideas would be most welcome and helpful.