Is it possible to DIY air-con cleaning? How?

Answer 1

I did this yesterday on a large system.
Get cleaning solution from an industrial supplier. Read the instructions fully and follow carefully. Spray the cleanser onto the condensor coil and let it foam. Use a garden hose to spray the cleanser and dirt out of the coils. Depending on accumulation you may need to repeat.
There is a similar cleanser and technique for the evaporator coils.