P1299 LINCOLN Possible Causes
What does this mean?
Low engine coolant
Faulty Thermostat Faulty Cylinder Head Temperature (CHT) Sensor
Cylinder Head Temperature Sensor harness is open or shorted
As to location:
https://www.2carpros.com/images/question_images/309520/original.jpg
What your describing: Initially started with heater blowing cold air, not getting hot.
Sounds like air entrapment.
Even though you say the coolant level is good , it is possible to have a air bubble in the system.
As to what caused it is another thing.
But I would suggest bleeding the system first and see if that does anything first.
Cylinder Head Temperature Sensor circuit poor electrical connection