lightening

I purchased a 2004 subaru legacy outback last fall. Upon the purchase this code (P030) appeared, and we negotiated a lower price and I was to fix the problem. I had the local mechanic change out the sensor, but it caused another sensor to go out. I decided to take it to a subaru dealer and they fixed the problem. Recently the real seal wen out on the transmission, and had to have that fixed, but in order to do so the exhaust had to be taken out. This revealed that the drive shaft needed to be replaced because of a non-servicable u-joint. I purchased a drive shaft from an auto salvage yard. We went out to get the car this evening and the service engine soon light had come on. Now we are back the this P030 code. Bank1 sensor . Any ideads why yhis sensor would go out again so soon would be greatly appreciated.