What should be checked first if an air conditioning system is not cooling properly?

When an air conditioning system is not cooling properly, the very first step should be to check the thermostat settings. The thermostat controls the operation of the air conditioning unit, dictating when it should turn on or off based on the desired temperature setting. If the thermostat is set incorrectly, such as being set to a higher temperature than the room or being in "off" or "fan only" mode, the system will not cool effectively or at all. Ensuring that the set temperature is lower than the current room temperature and that the unit is in cooling mode is essential for the system to function properly.

Checking the thermostat first can often quickly resolve the issue, clearing up any misunderstandings about system operation, without necessitating more complicated diagnoses or inspections of other components. This approach helps to streamline troubleshooting and ensures that simple user errors are handled before moving on to more technical checks like refrigerant levels, electrical connections, or air filter conditions, which may require more time and expertise.