Need help with sensor problem in 1999 chevy malibu

I have a 1999 Chevy Malibu. The check engine light has been on for about 6 months now. The original code was for the o2 sensor in front of the cat converter. I changed the o2 sensor and the light went out for about 2 weeks. Then it came back. The same code came up again. I then took my car to my regular mechanic who said that the code could be many things. He messed around with some wires with the map sensor and once again the light went away for a little bit, but it came back again. I am getting frustrated and if you have any idea what the problem could be please help me!
Thanks!

Do you spray water on your engine when you wash your car?

Your mechanic is right it can be a lots of things. You need to start with a good tune. Replace Plugs, filters, wires, PVC. Clean IAC, look for vaccuum leaks and use a fuel cleaner.