Nest is (with that many miles), I would suggest thermostat replacement. I know that you said the coolant level is good, but how good/effective is your coolant? Is the mixture is 50/50 or do you have more water than coolant, or too much coolant and not enough water. Use an antifreeze tester to check the strength of your coolant. Something like this one.
Another thing you may want to experiment, try switching between "fresh air" and "recirculate".
If the heater core is plugging up, you may not get enough heat either. The next thing is to perform antifreeze flush and refill, including the heater core.
Let us know.
Wednesday, January 23rd, 2019 AT 4:32 PM