PO128 Trouble Code

[ESOX]

Description is Coolant Thermostat (Below Thermostat Regulating Tempertue).

Do all I need to do is change the thermostat, or is there some senor hinden deep in the bowels of the truck that will take me hours to replace?

Thanks for your help, you guys have saved me thousands.
Truck now has 114K

[ESOX]

I found another post describing a similiar problem. I will let you know if replacing the T-Stat makes PO128 go away.