P0128 is Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Now I personally don't know much about the coolant system inside a car, but hopefully this will tip off someone who does. Maybe the thermostat is broken that reads the temp of the coolant? Is there any kind of heater in there that could be malfunctioning?
Unfortunately it doesn't sound as easy as an O2 sensor.