As a follow-up to this I queried my Cummins dealer. They thought the aftertreatment DPF in the muffler was partially plugged. A nearby truck shop wanted $750 to bake and clean the DPF. With some new gaskets and the help of a motorcycle jack and YouTube videos showing how to self clean a DPF I proceeded. It took a day or so for the DPF to full dry out using compressed air and a dryer on the airline...but I finally felt confident to re-install. What a pain that was. Anyway....having no way to test to see if the high temp light would come back....
I also noticed that my exhaust manifold was leaking at a couple of cylinders. Contacted a local shop to do the work for me...as i feared of broken bolts etc. the shop was great and had it repaired in a couple of days, even with sending out the manifold to be planned.
I have never seen the High Temp Exhaust fault since and I have towed on the way to Las Vegas though the mountains with good performance. It could be that the manifold leak was causing the high temp....not sure. And other than my time and $60 for new aftertreatment muffler gaskets, the cleaning of the DPF was free.
Happy guy....engine runs great.
2008 Fleetwood Bounder Diesel 38S, Flat tow-2015 Cherokee Trailhawk
Richard (KG5JAQ) & Margaret Kimball w/Shih-Tzu 'Charley'