Cat C9 Acert Engine
The majority of C9 engines produced (about 10,500) were 2004-2006 vintage starting with engine serial number 9DG. As mentioned the 2007-2010 serial numbered C9S are problematic with the after treatment system - separate DPF with diesel and igniter head built into unit. There were a sour 1,200 of these units.
I know of one 2008 C9 Allure that has had issues with a C9S C9.
Check engine serial number before buying.
Towr: 2007 Country Coach Allure 470 - 37 Sunset Bay Cummins 400 ISL, Coach #31563
Toad: 2016 F150 King Ranch - 3.5L EcoBoost 4x4 Supercrew (curb weight 4,775 lbs)
Toad: SOLD 2007 Jeep Grand Cherokee (yes, it has a Hemi) (curb weight 4,720 lbs)