2000 Corolla Check Engine Light

3 months ago, my check engine light came on in my 2000 Toyota Corolla (Approx. 60,000 miles) and I brought my car into the local mechanic. They told me my front oxygen sensor was bad. So I had it replaced, left the shop and two hours later the light came on again. Then they changed the sensor again and two weeks later, presto, light on again. They then blamed the problem on the part being an after market part and ordered a sensor from Toyota.
After some more time the light came on again except this time the code said something about the evaporator. The mechanic opened the hood and snapped a hose down tighter and then cleared the code. Soon thereafter again the light came on and then they told me I needed a mass airflow sensor. So I had that replaced and then went to pickup the car and it wouldn't start. The next morning they told me it wasn't in gear and was fine. two days later the battery died. I had that replaced at a different mechanic as the original mechanic shop was closed for vacation. I was hoping that changing the battery would prevent any ghost readings but, voila, this morning the light reappeared. I have already replaced two sensors, a battery, spent over $700 and three months later the light still comes on. What do think the problem may be? I look forward to your input.