Had a P0128 fault on my 2012 Big Horn, coolant temp below rating likley due to thermostat stuck open. Pulled the original thermostat (148K miles) and immediately could see the problem, some kind of rubber gasket had completely deteriorated and pulled off the top and bottom of the radiator side of stat leaving coolant the ability to flow through the little holes, also gummed up the bleeder valve. So, a couple questions;

1. Why would Mopar use a thermostat with this rubber feature, when coolants always warn that they are corrosive to rubber? The Duralast replacement stat I used is copper/SS, no rubber anywhere.
2. Where the heck did the loose rubber pieces go? Could it screw my water pump?

 

I lost the water pump on my Durango after only 80k and the thermostat was in the same condition as yours.

Pretty confident that rubber is what killed my water pump.