Other, less likely possibilities include:
* Blower motor itself: A failing blower motor could be drawing excessive current, causing the system to only work at full power to avoid blowing a fuse or damaging other components. However, this is less common than a resistor issue, as a completely failing motor would likely not work at all.
* Wiring issues: A short or break in the wiring harness related to the blower motor speed control could also cause this. This is less probable, but possible.
* Control panel issues: Though less likely, a faulty control panel could be sending only the high-speed signal. However, this is usually accompanied by other control malfunctions.
In summary: The blower motor resistor is the most likely culprit. Replacing it is a relatively straightforward repair (though the access to the resistor can vary depending on the model) and a much cheaper option than replacing the whole blower motor or other components. You can find diagrams and videos online showing how to access and replace the resistor for your specific model year.
