Male dogs like to urinate on vertical objects like posts, tires and, unfortunately, outdoor air conditioning units. And while the dog is primarily aiming for the appliance's outside casing, some urine can get inside the unit and touch the coils. Dog urine is corrosive against aluminum and copper, the metals most typically used for the fins and coils. Therefore, it is imperative to neutralize and remove any dog urine from the unit as soon as possible.
Turn off the unit and remove its back grill. Attach the garden hose to the spigot and then attach the spray head to the other end of the garden hose.
Spray the coil gently with a dog urine neutralizer such as Simple Solution, Anti-Icky Poo, Nature's Miracle, Zero-Odor, or Urine-Off! Try to spray the entire coil and let it sit for five minutes before moving on to Step 3. Also spray the fins, but from the outside and towards the unit. This is the same direction as the dog's urine encountered the fins.
Turn on the spigot and adjust the spray head to a list misting. Spray the entire coil gently, making sure to remove the neutralizer. Turn the water flow a bit harder, and then spray the fins with water from the inside out.
Repeat this process once per day until you cordon off the area with its own fence, or otherwise keep the dog away from the unit.