We need to suspect the blower motor controller is the issue on this type of problem. However, it could be the HVAC controller not sending the signal for the other speeds.
I am attaching the wiring diagram and we need to test the supply voltage coming into the blower motor controller. It should increase as the speed is increased.
I am attaching the process on how to replace the controller which is the most likely cause but we need to do this testing and go from there.
Here is a guide that will help with this:
Let us know what you find and what questions you have and we can go from there. Thanks
Images (Click to enlarge)
Monday, December 14th, 2020 AT 4:44 PM