My car is running too cold. Do you think its can be a Coolant Temp sensor?

What is your question?

service engine light is on. This happened a few months back and it fixrd itself. Same thing. The handheld diagnostics says the car is running too cold. Inturned off the AC and it went away. I just figured I had the AC overworked. Now its happening again. Could it be just a coolant temp sensor? And mayve a coolant flush could fix the issue? Or do you think it's the thermostat? It looks like a very hard job and I have to get the housing and everything. Brake light has been on for years but I dont know why. Nobody can figure it out. I replaced the brake pad sensors but the red brake light has been on since. Any ideas on this issue? I am trying to sell my car now. But want to fix these teo issues