I have a 2006 Chevy Aveo LT. I had my check engine light pop on a few weeks ago and had the codes read immediately-- P0420, P0700, and U0073. I know the P0420 is just my downstream 02 sensor as I was told this by a mechanic when it first popped on like 3 months ago but he had cleared the code. Last weekend we took our car to the Transmission shop to have the other two codes properly addressed and they said that there is a leak in the Water Pump and it needs replaced, and there is a leak in the oil pan and it needs replaced and there is a leak in our rear gasket between the motor and the transmission which needs resealed. Total was going to be about 1200-1600. Is there any piece of this I can do myself that might be easy to get to and change? (weekend job)? I was thinking the water pump and oil pan wouldnt be too hard. What do you know about replacing the O2 sensor? Is it pretty straightfoward? Any problems you guys run into or should I just run with that price and let the mechanics do it?

Thanks