Central air conditioner cannot be repaired by anyone but a professional. It has no user-serviceable parts. A window or room air conditioner does have parts that can be repaired without the need for a professional. However, because room air conditioners differ from model to model, brand-to-brand, repairing them will require that the owner’s manual be consulted.
Regular maintenance of a room air conditioner will keep little problems from turning into larger ones. Replace filters regularly; keep fans, coils and grilles (outside covers) clean and dust-free.
Clean the fan, using the necessary vacuum cleaner attachments to clean the blades and the housing, or wipe everything off with the damp rag.
Use the screwdriver to unscrew the selector switch from its housing, and check for scorch marks or other signs of damage. Take it with you to purchase a new one, and then install it according to the instructions included with the switch or provided in the owner’s manual.