To test the capacitor on a home air conditioning unit, shut off the power supply, discharge the unit and obtain the reading. If the reading is more than about six percent lower than the known rating, it is time to replace the capacitor.
When using a digital multimeter to test a capacitor, the job is easier if it has a capacitance setting. After shutting off the power, short out the two terminals with a screwdriver. This ensures the unit does not send out an electric shock, and it is now safe to disconnect the wires from the unit. Place the leads of the meter onto the terminal to obtain the reading.
If using an analog multimeter, set the meter to measure resistance before testing the capacitor. Turn off the power to the unit, discharge the capacitor and disconnect all of the wires connecting to it. With the meter at the highest resistance setting, place the leads on the capacitor terminals. This resistance reading must start at zero and then go up to the maximum. If it does not, replace the capacitor.
Sometimes, looking at the capacitor is enough to know if it is time to replace it. Signs it is time to replace the capacitor include seeing bulges in the unit or seeing an oily substance coming from the unit.