Carrier vector 1800 overheating!

Hello

Please advise as i have just about given up. I dont know much about these units so bare with me. I have a Vector 1800 that keeps overheating and auto shuts down after 12-14mins. i checked the expansion water bottle, opened the cylinder head to check the head gasket, nothing sinister to see with the naked eye. I will take it for pressure test next week just to be sure. It almost seems like there is very limited water circulation to cool the engine down or these is some form of blockage. She has been parked for almost 3 years so perhaps maybe there is a build up of rust?

much appreciate any advice fellas

did you check the water pump?

Thnx for the reply, Checked water pump no issues there

Thermostat work?
Rad clean?
Is it actually overheating or is the sensor faulty?
What about running clean water through the system?