One of the most common A/C failures is caused by a failing capacitor. A capacitor is simply a battery that has three terminals. You can buy a capacitor on Amazon for $20-30, but when a service person has to come out to replace one they will charge in the $150-300 range, depending on how much they want to take advantage of you! Capacitors are very easy to check and if their power is falling outside of the desired range, then they should be replaced as a maintenance item as the cost is really low and when they are not performing well, they negatively impact the lives of your compressor and fan which are very expensive to replace.
Most capacitors these day are dual capacitors as they kick on both the fan and the compressor. You should always use a capacitor that has the same or higher voltage as your system voltage. Never select one that has a lower voltage rating than your system voltage. The easiest way to know what capacitor you need is by looking at the one that is already in your A/C unit. There are two values you are looking for. First is the rating. You will see 35/5 uf or 45/5 uf. The 45 value is the rating for the compressor and the 5 value is the rating for the fan. Then there is a +/- rating value (i.e. +/-5%). When you test the capacitor using a multimeter, ideally the value is as close to nominal as possible. If your compressor rating is 45, it will still function if tested between 42.75 – 47.25. However, if you are measuring it on the low end (i.e. 43), you should just replace the capacitor.
To test the capacitor, get a multimeter and set it to Capacitance or OHM. Make sure the power to the unit is turned off. Usually there is a breaker right next to the AC unit that you can pull out / shut down. Next discharge the capacitor. If you touch a capacitor terminal with your hand and there is still energy within it, you can give yourself a nasty shock. To discharge it lay a metal screw driver with an insulated handle across the terminals.
To test first find the common terminal, which has C or 4 prongs and place the black probe of the tester on that one.
Then find the compressor terminal, which will have HERM or 3 prongs and simultaneously place the red probe on that terminal.
Check to see if the compressor falls into the proper range, that will be the higher of your two rating numbers (i.e. 45).
Next test the fan terminal by leaving the black probe on the common terminal and putting the red probe on the fan terminal which has FAN or just 1 prong.