Every homeowner these days is gung-ho to learn how to DIY everything. While that is certainly awesome (and less expensive!), there are just some things for which you still need a professional. For example, some air conditioning maintenance services cannot be completed by homeowners because of their technical nature, or because of regulations. Those services, as well as why you cannot do them, are as follows.
You cannot refill the refrigerant in your A/C because it is exceedingly complicated, very dangerous, and illegal. You have to have a professional license to purchase, carry, and dispense refrigerants. As a homeowner, it is very likely that you do not have such a license. Hence, you cannot even purchase refrigerant, much less refill your A/C unit on your own. This is just one of those services that only your HVAC technician can do.
While you can clean the exterior of the condenser box, you cannot remove the box cover/top from your A/C unit and clean inside. Very special cleaners are used to clean inside components to prevent deterioration and to prevent the wrong kinds of chemicals from accidentally mixing with refrigerants. Additionally, cleaning electrical components can prove lethal when you do not know what you are doing. Leave the annual cleaning of components to the technician.
Checking All Electrical Connections
The technician usually checks and repairs all electrical connections, too. There is as much wiring outside as in, so it all has to be checked thoroughly during the annual or semi-annual maintenance service call. If anything is amiss, the wiring has to be repaired or replaced. Someone who is not trained in HVAC or electrical work is not likely to spot such issues, and even less likely to know how to repair or replace electrical wiring safely.
Looking for Leaks in the System
The EPA regulates heating and air conditioning systems so that pollutants from these appliances do not enter the environment. As such, your HVAC technician is responsible for looking for and finding leaks. He or she knows what the leaks will look like, and he/she can determine just how serious those leaks are. Additionally, even if you could recognize leaks in your air conditioning system, you cannot patch them as you would with ordinary plumbing. Attempting to do so can make the leaks worse, and it can result in personal injuries to your skin. Leave it to the pro.