What does a blower resistor do?
Blower resistors control the fan speed of the cabin blower. The fan speed can be changed either by switching the blower resistor resistance mechanically by using a rotating lever (manual A/C system) or electronically by the air conditioning system (automatic A/C system).
How to tell if you need a new one...
A typical indication of a faulty resistor is when the speed of the fan cannot be controlled – e.g. it only runs at one speed or is not running at all. Either the resistor is burned out (change it) or the electrical power is irregular (measure with a multi-meter).
However, we recommend that you always replace the resistor when you install a new blower.