2006 GMC Sierra P0128 After Cooling Repairs: Check the Radiator, Not Just the Thermostat
Persistent under-temperature can come from an incorrect coolant path. This Sierra had a replacement radiator missing a restriction in the surge-tank connection.
Tag GMC Sierra
2022 GMC Sierra P129F After a New Module: Test the Fuel Supply Before Buying Again
A replacement control module did not cure this Sierra. Low lift-pump pressure led to tank cleaning and pump replacement; the lesson is to separate electrical command from actual fuel delivery.