How to Test the Starter Capacitor in a Furnace

Switch the power off to your furnace. If you attempt to test a capacitor while power is running to the furnace, you will receive a potentially lethal shock resulting in death or serious injury.

Remove the front panel on your furnace, then remove the blower motor panel. Inside the blower compartment you will find the capacitor.

Touch one end of a rubber-insulated screwdriver on one of the capacitor’s terminals. Lean the screwdriver over toward the other lead so that the screwdriver touches the other terminal. Doing so will short the terminals and discharge the capacitor. Always hold the screwdriver by the insulated handle. You may hear a popping sound as the capacitor discharges. This is normal.

Remove the capacitor from the furnace by disconnecting all wires that attach to it. The type of capacitor you have is determined by the terminals. A run capacitor is small and rectangular-like. It has two terminals at the top of the capacitor. A dual-run capacitor is larger and has three terminals.

Prepare the test by turning on your multimeter or capacitor tester. Your multimeter must be able to test for microfarads. Some multimeters cannot test for microfarads. If yours cannot, use a capacitor tester. If using a capacitor tester, set the microfarad range to equal the microfarad rating found on the front label of the capacitor. For example, a run capacitor may have “5.0” as the microfarad rating. A dual-run capacitor will have two ratings -- one for the herm terminal and one for the fan.

Test the run capacitor by placing the black negative probe from your multimeter or capacitor tester on one of the terminals. Place the red positive probe on the other terminal and wait for a reading to appear. If the reading matches the microfarad rating found on the capacitor’s label -- within the given plus and minus range, which is also listed -- it’s properly functioning. If it the reading does not match the rating, it needs to be replaced.

Test a dual-run capacitor by identifying the terminals. Each terminal is identified by a small label. If you cannot see the label, the “C” or common terminal is the one with the most connections, while the fan has the least amount of connections. The herm terminal has three connections.

To test the fan terminal, place the black negative probe on the common terminal and the red probe on the fan terminal. Compare the reading with the listed microfarad rating. Test the herm terminal by moving the red probe to the herm terminal. Keep the black probe on the common terminal. Compare the reading with the rating.