Code P0128, Coolant Thermostat (coolant temperature below thermostat regulating temperature)?

One of the few codes that actually points to a part. P0128 is 99% a bad thermostat. Basically, it means that the ECU has a timer inside and once the engine is started and being monitored it should be up to operating temperature with a set time frame, the code means that it isn't and there are usually two reasons, one is a bad thermostat, the other is the cooling fans stuck on. Since you don't mention it sounding like a plane it's likely the T-Stat.
The repair is to drain the coolant, remove parts to heat to the thermostat housing, remove the housing with the thermostat attached. Replace it with the new housing and put it back together. Fill with coolant and burp the system to get the air out. Then verify that the car is getting up to operating temperature.