Engine Light on, kicked EGR valve code, changed the valve and light is back on?

Now says EGR flow, what could cause this? I have a 1998 Chevy Lumina with a 3.1 engine. Just after passing inspection my check engine light came on, so I wen to my mechanic, he put it on a computer and it kicked a code for the EGR valve. Yesterday we replaced the valve, he ran the test again and it said there were no codes. Today the engine light is back on, because my mechanic is busy today with his sons graduation, I took the car to the parts place were I buy my parts hand then run another scan.

The man told me that the code is reading EGR flow. I am wondering what the chances are that the part is defective or there is another problem. Of course he told me he does not believe the part is defective, but why wouldn't he being an employee of the business I bought the part from. I am wondering what else could cause this code, seeing the part is brand new.