So, The only reason I am writing this is because when I had this issue and nothing I read was working. First I made sure all the hoses were attached. I tried replacing the EGR valve itself. Still got the code after 30+ or - miles. I tried replacing the EGR solenoid. Still got the code after 30+ or - miles. I tried replacing all the hoses. Still got the code after 30+ or - miles. I tried clearing out the EGR system with a coat hanger. Still got the code after 30+ or - miles. You can see a pattern here.

I had tried everything I read about and nothing fixed the issue. So, I decided to check where the white and yellow stiped hoses attached to the throttle body to make sure they were flowing. I checked the yellow hose and that one was free flowing. Then I checked the white hose and nothing was flowing through it. I removed the throttle body and checked again. Nothing. So, I sprayed the hose connection with brake cleaner. Some black stuff came out but was still not flowing. I found a small wire and tried to stuff it through this little hole on the inside of the throttle body.


Out pops a chunk of black stuff. After I cleared this to where it was free flowing I re-installed the throttle body, cleared the code and it worked! No more P0401!

I hope this helps someone who has tried everything else like I did. BTW this was on an 2003 Evo 8 with 140k miles on it.

 

Nice work I'm glad your persistence paid off, and we can all learn from it. Thanks for sharing

 

Nice find!