Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Can you give me engine size? If unsure, I can use the 8th digit of your VIN# XXXXX identify.
Reposition the inlet hose clamp at the water outlet.
Remove the thermostat.
Clean the sealing surface of the water outlet.
Clean the sealing surface of the intake manifold.
Actually, more commonly with the P0128 is a faulty ECT (Engine Coolant Temp) Sensor.
Not a bad job at all. Simply remove the electrical connector, and unscrew the sensor.