I have a similar problem. I have P0401 and code P0421 pop off and I have swapped out both the egr valve and the egr solenoid. I reset the computer and drove around for 4 days before the check engine can back on. If anyone has any suggestions or solutions please message back.

 

Hello Forum Enthusiasts!
I have a 2003 Mitsubishi Outlander FWD 2.4L DOHC and have been seeing the p0401 code and p0420. I solved the p0420 bought a new catalyst. I am trying to solve a P0401 now I used compressed air and Carb cleaner to clear gunk out of the intake and exhaust ports going to the EGR Value. I cleaned out the EGR value itself with Carb Cleaner also to get a lot of gunk out of it and now the tip inside the EGR value is cleaned out. I tested the EGR Value with a vacuum line gauge. Its functionally opening and closing as it should. The EGR solenoid I tested with a Multimeter and it is reading properly. The vacuum lines are all checked and connected properly and free from cracks or damage.
The strange part that I cannot figure out is at idle the vacuum line is not getting suction going from the EGR value, to the EGR solenoid unless you cover the intake opening on the air box. If the air box opening is not covered that vacuum line going from EGR value to EGR solenoid has no suction at all.
I welcome your thoughts, opinions, and support, and I thank you all in advance

~Andrew~