PO 128? Coolant temp sensor or thermostat

[NWReynolds]

2011 Subaru Impreza 2.5 liter.
I’ve owned the car for 1-1/2 years no issues. Yesterday cel and flashing cruise light and trac control light. Parts store read the codes and I’m at po128 the guy said either a thermostat or engine coolant sensor. I bought both parts and started my way home well car began to run sluggish and was jerking here and there. Then heater just stopped working then came back on this happened like 3-4 times. I was happy to see the car didn’t give the red coolant light I assumed it’s red since it’s blue when I warm up the car so not overheating. I made it home and was very relieved to do so. I just had surgery and not quite ready to crawl underneath to drain and replace the thermostat so I’m just going to park it until I can do all the work. Looking for some suggestions and guidance since I’m new to Subaru’s. Any help would be appreciated very much.

 

[Ecmbuster]

This link will help you understand why P0128 was recorded in fault memory.
https://www.samarins.com/diagnose/p0128.html
The TSB may also be a guide.
Your current problem may also be attributed to an "air lock" in the cooling system.
If unsure, have a professional inspect the cooling system.
Cooling system temperatures can reach 100 Degrees C or greater.