Heater is always on

Hello,

So, when you turn on the A/C, does the compressor turn on?
If the A/C compressor is working properly but you only get hot air, then it could be due to a faulty blend door actuator.
The blend door actuator is what controls the hot and cold air flow/mix.
A faulty actuator can fail on the hot or the cold side. That will lead to only hot or cold air out of the vent.

The blend door actuator is located behind the dash on the HVAC box.

https://www.2carpros.com/articles/replace-blend-door-motor

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.