One of the most neglected parts of a air condition system is the Condensing coil. yet nothing affects the efficiency of the system than the condensing coil. Think about it, All the heat that is picked up from the house is expelled for from the condensing unit. But unlike the Indoor coil which is filtered the coil is exposed to all the elements plus dust and dirt from cutting grass and so on.
Cleaning the Coils is a excellent way of extending the life but the proper way is essential as you can do more harm than good.
FIRST AND FORMOST. DISCONNECT POWER AT THE BREAKER AND TURN OFF AIRHANDLER.
Remove condensing fan and from top of the unit use a garden hose to wash unit from inside out , A little detergent goes a long way, Coil cleaners are available from most home improvement stores. It is critical that not to pressure wash the coil as you can damage it. Many times I have seen were the coils are so damaged that the from pressure washing that the A/C needs to be replace. a slow rinse is better.
If you don’t want to do all this a trained professional can. Reassemble the unit. and your good to go.