Quick Scoop

To check a capacitor, the safest and most useful method is to disconnect it, discharge it, then measure its capacitance with a multimeter and compare the reading to the value printed on the case. A healthy capacitor should usually read close to its rated value within its tolerance, while bulging, leaking, or a reading far off spec are strong signs it needs replacement.

What to do

  1. Turn off power and remove the capacitor from the circuit if possible. Testing in-circuit can give misleading results.
  1. Discharge the capacitor before touching the terminals. Stored energy can remain even after power is off.
  1. Set your meter to capacitance mode and connect the probes to the capacitor terminals.
  1. Compare the measured value to the label on the capacitor. If it is well outside the stated range, it is likely bad.

Extra checks

  • Look for swelling, seepage, or physical damage on the casing.
  • If your meter does not measure capacitance, an ohms test can still help: a good capacitor often shows a brief low resistance that rises toward infinity as it charges.
  • For HVAC capacitors, people commonly replace them if they fail the capacitance test or show visible damage.

Simple example

If a capacitor is labeled 50 µF and your meter reads 39 µF, that is usually a sign it is weak or failing, especially if the label tolerance does not allow that much drop.

Safety note

Capacitors can hold charge after unplugging, so avoid touching the terminals until you have discharged it safely.