Joe, this is from a quick Google using 2005 f250 code 404 as the criteria. "P0404 FORD: Code Meaning, Causes, Symptoms, & Tech Notes (With Video)" https://www.engine-codes.com/p0404_ford.html#causes

Sounds like your EGR is hanging open, affecting the EGR flow rate. Hope this helps, or at least gives you a link for mere ideas.