Try change'n your thermostat... worked for me..
2004 gmc envoy with code p0128
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Engine coolant Sensor is bad. I have the same error on my 2005.
Trouble code P0128 means: ECT Sensor Below Thermostat Regulating Temperature
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Trouble code P0128 means: ECT Sensor Below Thermostat Regulating Temperature
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)
Trouble code P0128 means: ECT Sensor Below Thermostat Regulating Temperature
Trouble code P0128 means:Coolant thermostat (coolant temp below thermostat regulating temp)