The most common symptom of a faulty blower motor resistor is a blower fan that only works on one speed, usually high. This can be annoying, but troubleshooting is easy. If you start at the blower motor, and check resistance in the circuit, you will find the problem in no time.
Power supplied from the fuse panel passes through the switch to the blower resistor assembly. The switch supplies ground to the different resistors in the assembly, depending on the speed selected. The added resistance in the circuit slows down or speeds up the blower motor as needed.
Locate the fuse for the blower motor in the owners/service manual. Test the fuse, with the ignition key on, using the 12-volt test light. Attach the clip lead of the test light to a good ground like a bolt screwed into metal, and touch the sharp probe end to the terminals on the fuse. An indication of power at both terminals of the fuse determines that the fuse is good. If one of the terminals has power and the other does not, the fuse is bad. Replace the fuse as needed.
Unplug the blower motor and test for power using the 12-volt test light in the same manner as explained in Step 1. Testing at the motor itself eliminates the possibility of confusion caused by complicated electronic blower controls found in Electronic Climate Control systems. Test for power at all the blower speeds by moving the blower speed switch through all positions.
A faulty resistor loses power at different positions on the switch. High blower speed has direct power to the motor from the switch. If there is no power indicated on high blower speed, the problem is most likely the switch or the fuse. If there is power indicated on high blower speed, but not low or mid range, the problem is the blower resistor.
Test for ground at the blower motor if there is power indicated at all ranges, but the motor does not function. Most blower motors ground through the case of the motor when it is screwed into place or by a small ground strap from the case to the body of the car. Over time, the ground connection can become corroded or loose, causing the motor to function intermittently.