EGR code P0401 insufficient flow?

Car doesn’t have symptoms that I’ve read come with an EGR issue. Runs pretty well and I think mileage is ok for the age of the car 25mpg ish. I was told to replace the valve so I went through hassle trying to find the part. Tried pick n pull replacement, cleaned it and the original, and installed a brand new one. None of that fixed the check engine light. Cleaning and visual inspection of valve and manifold area indicate low carbon deposits and easy movement of valve.

I’ve read lots since and none of the described diagnostic testing has been done and from what I’ve read, I’m coming to the conclusion that the EGR valve itself is not usually the cause of the EGR codes.

So, if your mechanic doesn’t know or do the tests, how do you actually find the cause or identify/find a mechanic that will actually diagnose instead of replacing this part replace that part.

If it is damaging the engine every time you drive it would be nice to find a way to fix it.