Heater blows warm air on low speed?

Hello,

This could be due to a faulty blend door actuator.
The blend door actuator is what changes the air flow from hot to cold and cold to hot.

Over time the actuator cab become worn and fail in one side or another (hot or cold).
It is possible that the actuator is stuck on the partial warm setting and as the fan speed increases it draws more air from the cool side.

The blend door actuator is located under the dahs in the HVAC module.

Here is a helpful guide:

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.