The blower motor is really the heart of the HVAC system. Without it, you don't have airflow in the system. If your blower motor goes out, you won't have heat or AC. So if you turn on your HVAC and you don’t hear anything, you might start by checking out the blower motor operation. Blower motor circuits are often similar in design. You have the motor itself, a switch to run it, and some form of resistor or transistor assembly that controls the blower speed. If you study the problem, it can tell you a lot about what might be wrong. When the blower is on high, it's getting full battery voltage so it can run at full speed. If you select some other speed, the current flows through a resistor or transistor assembly that takes away some of the feed voltage. If the blower receives less voltage, it will run slower.
With this in mind, if your blower only works on high speed, what do you think the problem might be? That's right: the blower resistor or transistor. It's a common problem and many times is easy to fix. The transistor or resistor assembly is often mounted in the HVAC system and is accessible from the outside. If the blower doesn't work at all, try tapping the outside of it with the palm of your hand. This might cause the motor to start working. If this is the case, you likely have a bad blower motor or a loose connection somewhere. My money is on a bad blower motor; they do wear out from time to time. It's a good idea to check for power and ground at the motor first before you replace it. It could be some other issue that causes it not to work, as you'll see in this video.