Toyota - Camry :: Check Engine Light Illuminated / Oxygen Sensor?

I own a 2000 Toyota Camry that was given to me by my father. The check engine light illuminated last year so I brought the car in to my mechanic. He said it was the o2 sensor and changed it. That lasted about a month , and the light went on again. This time it had to do with some valves that get stuck open (or closed) but also had to do with the amount of oxygen that goes into the carburetor . I asked my Dad if he had this problem, and he told me that the engine light was illuminated often, that he had brought the car in about 6 times, but nothing was found. Previously I owned a 1998 Camry, and had the same problem; the check engine light illuminates, the diagnosis is the oxygen sensor, that gets changed, the light goes on again, this valve thing gets changed but the light goes on again in a month. I can't get the car inspected with the engine light illuminated. The light always goes back on but nothing else is found wrong with the vehicle. I really know nothing about cars.