You expect your heater/air conditioner to work like it should.  You have a control for temperature and one for fan speed.  You even have a control for what vents the air comes out of. 

Don't be surprised one day if your blower fan develops a mind of its own and starts going crazy.  Most of the time, you may find that it starts blowing at full speed, and nothing you do to try to control it does any good.  This is what may be happening.

Your blower motor has an electronic component called a resistor.  It does what its name says; it offers resistance.  When you want the fan to run more slowly, you turn the fan speed down.  That resistor accomplishes that by turning its resistance up.  When the resistor fails, the power has nothing to slow it and the fan speeds up. 

It's a small part and can fail due to age or corrosion.  It's usually not an expensive part, either, but it's often found in a location that's not that easy for the technician to get to.  That means labor costs will vary depending on the design of your vehicle. 

Occasionally, a faulty resistor can cause the blower motor not to work at all or only partially come on.  But other things can cause that as well, such as a faulty fan switch or vent control. 

This is where a technician's training comes in.  Special equipment can track down precisely where the issue is so you can be assured the correct part is being replaced.

It's just not pleasant when the blower motor isn't following orders.  Have your service facility check it out so you can be the blower's boss, like it should be. 

Perfect Tune Auto Repair
5434 S. Old US Highway 23
Brighton, Michigan 48116
810.227.9903
https://www.perfecttuneautorepair.com