The check engine keeps coming on for a couple a days and then goes back out. On a diagnostic check the codes are P0404 and P0406. These are not listed in my shop book. It goes from P0403 to P0420. In the owners manual it says to check the gas cap, make sure it clicks 3 times in which I have done. After a few starts it goes off but then before I get gas again it will do it again. What else might I look at? Any help would be appreciated, since our Dodge dealer has went out of business. This is a 3.8V6.

 

Those are EGR codes
You likely need a new EGR valve.
Fairly common failed part.