Looking to replace my present thermostat. I have 135k on the vehicle and live in Colorado (thin air). Should I use a 180 or stick with the 203 degree thermostat. Because of the age and thin air I’m think 180. Any thoughts?

 

I think I found my answer:

The P0128 code means that the engine coolant is not getting warm enough, fast enough. In other words, there is a problem with the engine operating temperature not being reached within a certain amount of time. The Engine Control Module (ECM) determines this length of time based on ambient temperature.