Dpf cleaned and mileage dropped 1 mpg

[Trucker750]

I have a 2012 cascadia with a cummins 11.9 400 hp it has 620k on it. The dpf filter was cleaned and mpg seemed fine, but then 250 miles down the road the outlet NOx sensor went out and was about 1500 miles of reduced mpg. From 6.2 to 5.8 mpg. The sensor was replaced, but instead of mileage going back up, it went down even more to 5.2. I'm losing 1 mpg now and dont know why... no codes, so I need someone who might know what's happened???