Evaporator AC have a certain age to keep it functioning optimally. Usually age can reach about 5 years. When it reaches this age, there will be certain problems in the form of a pungent smell like burnt smell. Then, less cold air and also appeared hissing sounds continuously.
It is possible also, the evaporator is blocked by dust because rarely cleaned. But, it is mostly due to the age factor. Indeed, there is a special sign of the emergence of this sign, there is a freon leak. If these signs occur, it would not hurt to check the air condition in a special workshop.