why am i still having trouble with the scan code P0401 insufficient flow to the egr?

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I had my engine came on. I did a engine scan. I have tested the egr,the vacuum switching valve, and the egr modulator. And Everything was good. I have made sure that I didn't have any sort of blockage. Then I checked the components agian just to be sure that everything was good before reinstaling agian. The vacuum switching valve was giving me a problem. So I replaced the valve. Cleared the code and disconnected the battery to clear all sensors. Then about 13 miles I had the engine light come back on. So i decided to change the vsv, the egr ,and the egr modulator. I clearded the code and disconnected the battery again. And took it for a test drive I saw that i was getting better gas mileage and that told me that it wasn't the map sensor, and i went exactly 52 miles and the engine light came back on. So I done another diagnostics scan and the popped up as P0401 insuffient flow to the egr. I am wondering if i had missed anything or if there is another component that I don't know about. I know the egr is a pretty basic system. But if anyone can help me that would be great. I needed to get this car inspected by the end of febuary and of course it's my wife's car. So that is a problem for me. So anyone with ideas would be a great help. Thanks