coolant fan not working, are there sensors or fuses or something?

Hello,

When antifreeze leaks but you cannot find a leak, it is typically leaks out of the radiator overflow or the radiator cap.
You could have a faulty cap that is letting out the coolant as it warms up. Which is impacting the heater performance.
You can test the pressure of the radiator cap and make sure if it is in good working condition.
If the cap does not hold its pressure the coolant will leak out as the engine gets warm and the pressure builds.
Also you can pressure check the cooling system, this might help you find where the coolant is coming out of.

Here is a helpful video:

https://youtu.be/W8kra65m15c

https://www.2carpros.com/articles/radiator-pressure-test

Another reason the heater might not get hot enough if you have a stuck open thermostat. A stuck open thermostat will not allow the engine to reach its operating temperature.

https://www.2carpros.com/articles/how-an-engine-thermostat-works

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.