Check Engine light will not turn off!!?

I own a 2001 Chevy Monte Carlo LS. Recently the check engine light turn on. I took it to the mechanic and it turns out it was the EGR valve. Well i replaced it. A few days later it turned back on, so i took the car back to the mechanic. This time it was the O2 sensor. I got a new one. Then a few days later the light turned back on. It was the EGR valve again. I got it replace AGAIN, but in few days the light came back on. It keeps telling me that its the EGR valve, but the mechanic says that it running fine and that the EGR valve is working properly. Now he wants to replace the computer on the car. Does some one have an idea of what the problem might be before i replace the computer on the car.