Killing mold on an A/C unit is not as difficult as you might think. However, cleaning out the unit to ensure that the mold is dead is slightly more difficult. Since most mold in air conditioners grows on damp dirt and dust that has collected in the unit, you must be sure to remove all dust particles from the area. Fortunately, it only requires a little time and some common household cleaning supplies to whip your air conditioner into shape and kill mold on an A/C unit.
Mix up your cleaning solution. In the spray bottle, combine 2 cups of water, a cup of bleach and 4 drops of liquid dish detergent. Swirl them to mix them up, but do not shake the bottle to form suds. Set the bottle to "spray" when you are done.
Remove the air filter. Spray it lightly with the mold-killing spray before you start working to prevent mold spores from launching into the air. You should be wearing your mask at this time. The filter may pop out, or you may need to unscrew a few screws to get it off. If it is disposable, then seal it in a plastic bag and throw it away. If it is reusable, then submerge it in 3 inches of hot water in the sink along with a cup of bleach and 3 drops of dish detergent. Let it soak for 15 minutes while you are working on the rest of the air conditioner, then rinse and remove it from the water and allow it to dry while you finish.
Remove the A/C unit grill. You will probably need to unscrew several screws and then spring the grill by pulling gently forward while pushing down. Set the grill to the side for additional cleaning in a minute.
Spray the interior with the mold-killing spray. This will weigh down the spores and should kill off the mold.
Wipe out the interior of the unit. Use the cleaning rags and the plastic scrubber for tough spots. Be sure that you have removed every particle of dirt from the interior. When you have finished, spray the entire area down again and wipe it out once more to ensure that all mold spores and dirt that could be growing mold are gone.
Scrub the grill. Use a liberal amount of your bleach spray and the plastic scrubber or the scrub brush to clean off the grill. Often dust that has made it past the filter will collect on the grill, so be sure to get everything off the grill and dry it thoroughly to prevent new growth.
Put the air conditioner back together. Replace the grill and screw it back into place. Pop the dry filter back into its position. If you like, you can mist the filter with a little more bleach to impede future mold, but do not saturate the filter. A fine mist is plenty.