A faulty blower motor resistor will cause the heat and air conditioning fan controls to malfunction. Typically, the fan won't work at all, or it will only come on when the switch is placed in the high-speed position. Testing the blower motor resistor with a multimeter is a simple way to determine if you need to replace the resistor. If the resistor is good, the problem likely lies with the blower motor itself.
Remove the negative cable from the vehicle's battery. Loosen the nut with a wrench until you can pull the cable off. Tuck the cable behind the battery so it no longer touches the battery post.
Open the door on the passenger's side of the vehicle. Duck your head underneath the dash and look at the ventilation tubes that dump at the passenger's feet. Follow them until they hit a large protruding black canister. This is the blower motor.
Remove the wiring harness from the blower motor. The part that the harness plugs into is the resistor box. With a Phillips screwdriver, take out the screws holding the resistor box in place. Pull it off of the motor.
Pry the lid off of the resistor box with a flathead screwdriver, exposing several resistors inside the box. Clip a multimeter to each leg of the first resistor and turn the knob to the "Ohms" setting, which is normally signified by an omega sign. Make a note of the recording on the screen.
Write down the coloured bands on the resistor and use a resistor code calculator to determine how many ohms of resistance that particular resistor is supposed to provide. The bands follow a colour coding system. For example, two red bands and one orange band means the resistor is supposed to provide 22,000 ohms of resistance.
Compare the multimeter reading to the resistor's ohm rating, as determined by the calculator. If it is off by more than 5%, the resistor is bad. Continue checking each resistor in the resistor box. If any of them are bad, you should replace the entire box with a new unit.
Reinstall the lid on resistor box. Reattach the harness on the box, then secure the box on the motor with the screws. Reconnect the negative cable to the battery. Test the blower to see if it works properly.