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Hi, and welcome to FTE and the 4.2/3.9 Forum.
There have been several posts here over the years regarding P0401 (and P0402) codes.
While in the 4.2 forum, Click on "Search this forum" the top and type "P0401" in the search box. You'll get plenty to read.
I seem to remember 2 most common causes:
bad DPFE sensor
or
clogged EGR ports
Best of luck with repairs and let us know what you find as the cause.
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