Must not be to bad. Normally when you get this code you will notice the temperature is not normal in the interior.
DTC P0128 is designed to detect for a stuck open thermostat
Google “Duramax P0128” will explain this code.
Since yours is a 2011 if you haven’t done a coolant flush yet this would be a good time if your going to put in new thermostats.
If you plan on doing this yourself there is a DIY on here also lots of YouTube videos.
Last edited by Rvn4me; 12-14-2018 at 02:51 PM.