What you least expect on a hot day is an AC that does not cool. There can be several reasons behind AC not cooling problem, some of which are technical faults and some are non- technical.
Most common reasons behind AC not cooling up to mark are as follows:
Clogged air filter
If the air conditioner does not cool, the air filter might be clogged. You will know this when you open the AC and see a crust or layer of dust on the air filters. The colour of the air filters is also a measure of its cleanliness. In such case, air cannot flow through which greatly reduces the cooling capacity and efficiency of the air conditioner. Service your AC periodically to avoid this.
If the air conditioner is not cold enough the condenser coils may be dirty and loaded with debris. In an AC, the condenser is like a radiator that dissipates heat (which was removed from the air). The condenser coil must be clean to dissipate heat outside. As the coils get dirtier the air conditioner becomes less efficient, which makes it work harder to cool the room.
The refrigerant circulates through the evaporator coils and cools the air flowing over it and eventually sends cool air into the room. So when the level of this refrigerant becomes insufficient due to multiple reasons like a leak, a crevice, etc. the AC fails to cool.
A compressor is the heart of any air conditioning system. If the air conditioner does not cool, the compressor might be defective and your AC might be blowing only air through the fan motor. The compressor is a motor which compresses the refrigerant or coolant and circulates the refrigerant through the evaporator and condenser coils. If the compressor goes bad the cooling cycle does not start and the AC fails to cool.
The other possible technical reasons behind AC not cooling can be a faulty run capacitor of the compressor, defective control board, bad thermistor, choked capillary, faulty motor, etc.
High atmospheric temperature
An AC has an optimum operating temperature (surrounding temperature) at which it can work most effectively. If the temperature shoots up beyond the optimum temperature limit, the efficiency of the air conditioner goes down. Sometime it is so hot outside that a stage is reached where the compressor stops working completely.
A common mistake people make is that they buy an AC based on their budget and not basis the requirement of the room they intend to cool. An Ac of small tonnage in a huge room will always find it hard to cool the room in peak summers. The size of the AC (tonnage) should be according to the size of the room.
Faulty AC Remote
There can also be a situation when the remote fails to send temperature change signals to AC unit. It could be a simple replacement of remote or the batteries inside the remote.