Hello, Dave here, contributing to RepairPal and providing expert advice since 2010.
There are several possible causes for your heat not working. Since you said the air blows cold, it sounds like the blower works. The heat comes from hot coolant circulated from the engine that goes into the heater core behind the dash. The fan blows through the heater core when hot air is commanded by the blend doors in the dash that direct hot and cold air depending on what is desired.
The first thing we need to find out is – is there hot coolant going into the heater core? If there is hot coolant going in and coming out of the heater core, both hoses that go to the firewall at the back of the engine compartment will be hot, extremely hot. Almost to the point where you can't touch them. This is due to 200°F coolant when the engine is at operating temperature.
If it is not hot, or is warm but not super hot – there could be an engine temperature issue. If it is super hot then you need to look at what is directing the airflow behind the dash, in the way of temperature blend doors and the motors that turn them.
Going back to the engine not being hot enough, usually the engine thermostat might be opening too soon or sticking open, preventing the engine from getting to the proper temperature on an ongoing basis. I know you mentioned replacing the thermostat, but I would just ensure it's the correct temperature thermostat for your vehicle.
You could also have an air pocket in the cooling system for whatever reason. This is very important to inspect and resolve when opening the cooling system -- when the thermostat was replaced. Extra time and effort must be employed to purge cooling system air out after repairs.
If you'd like assistance looking at these various areas, reach out to a local shop near you with experience in diagnosing this kind of thing.
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