Run Capacitor Diagnostic

Testing the run capacitor

Use this procedure to diagnose if the inducer or blower motor run capacitor is the source of your furnace problem — after you’ve done basic troubleshooting and the blower motor or inducer motor diagnostic.

Note: This procedure does not apply to ECM blower or inducer motors.

Tools needed

• standard multimeter with setting to measure capacitance in microfarads ‘µF’

Testing procedure

1. Turn off the power to the furnace.
2. Remove the screw(s) holding the run capacitor to the blower motor housing and unmount it.
3. You will notice two identical male spade connector ports. The wires are usually in the form or a female spade connector and are friction fit. Note the orientation of how the wires were connected so they can be reinstalled in the same location.
4. Disconnect both wires from the top of the run capacitor. They should disconnect with a firm, steady tug.
5. Set your multimeter to measure capacitance in ‘µF’ (microfarads).
6. Contact the multimeter probes to the male connection ports where the wires were connected. One probe to each port.
7. Hold the probes in place and record the reading on the multimeter. (Some multimeters require probe contact for at least five seconds to show a reading.)
8. Make note of the factory voltage and µF ratings, which are always stamped on the side of the capacitor.
9. Inspect the capacitor. If it’s damaged or bulging from the middle unnaturally, you should replace it (even if it’s operating within specifications).
10. Once you are finished with diagnosis/replacement, reconnect the run capacitor and remount it.
11. Turn on the power to the furnace.

Interpreting the results

Most run capacitors for residential furnaces are rated for 350 – 440 volts and 5 – 15 µF (5 µF , 7.5 µF, 10 µF, and 15 µF are common).

Compare your measurements with the factory ratings. If your measurements are more than 5% above or below the factory rating, you should replace the run capacitor.

Once you’ve replaced the run capacitor, the performance of the motor should improve. It should ramp up to speed more quickly and make less noise.

Note: If you’ve replaced the run capacitor and the blower motor or inducer motor is still performing poorly or making noise, you should consider replacing it.