Testing a capacitor
NOTE: Before you test this component, make sure your microwave is unplugged, and that you have discharged the capacitor.
Before you begin this test, first examine the state of your capacitor. Does it appear to be burned in any way? Is there an oily film present on the component?
With your microwave's cabinet opened up, take careful note of which wires attach to the terminals of your capacitor. You may want to label them with masking tape so that you know which wires are to be reconnected where. Also, take note as to whether or not your microwave uses a bleeder resistor. If your model does have one, you do not need to remove it, but keep in mind that some of your readings will reflect the meg-ohm resistance of the bleeder resistor.
Set your ohmmeter to the highest resistance scale. Then place each of the meter's probes to one terminal. You should receive a reading of infinity, or you should receive the value of the resistor. Now you can reverse the probes so that they are each touching the other terminal. This should result in the meter momentarily leaning toward the zero mark, but then drifting back to infinity. Reverse the meter's probes yet again. The same pattern should occur again.
Now touch one meter probe to a capacitor terminal, and the other probe to the capacitor's metal casing. You should receive a reading of infinity. Now touch the same meter probe to the other terminal, while keeping the other probe touching the metal casing. Again, you should receive a reading of infinity. However, if an internal diode is present, then the reading you receive could be reflective of the diode's forward bias resistance. You might want to take a look at our diode testing page.
If your test does not produce these results, or if your initial visual inspection of the capacitor reveals signs of damage, replace your capacitor.