I am sure other people addressed this, but just in case ..
The main things that it could be ..
- Make sure your indoor filters (there may be more than one) have been replaced recently. Some houses have more dust, dirt, sand, etc. than others and require more frequent filter replacements. Houses like that tend to have a harder problem with the more expensive higher density filters so you may have to use mid or lower quality filters. As the filters filter particles, the outside unit has to work harder to pull air in through the filter.
- Your outside unit could low on freon, but that would mean there is a leak somewhere which needs to be located and sealed.
- Your outside unit may need cleaning. If there are trees, bushes, etc. nearby, you should clean it at least once per year.
- There is a blown fuse or burned/disconnected wire in the outside unit.
- Your air compressor is bad or going bad.
- Your evaporator is bad or going bad.
- The outside unit is not powerful enough to cool the square footage of your house. It may work fine until the temperature reaches a certain level.
- The outside unit can freeze (freeze as in ice) up if it runs too long without a break, but this would typically be a symptom of one of the above issues rather than the primary cause of the problem.
Do not assume just because the guy had the air conditioner replaced in 2015 that he replaced it with a new unit. I have dealt with air conditioner repair services that have stocks of used air conditioner units from other client's of theirs who upgraded to more powerful units. They will then offer them to people who need air conditioner replacements at much lower costs which is mostly pure profit for them since they usually do not pay for those units when their other clients upgraded. Given that you just bought the house, it is possible that the previous owner chose to replace the unit with a used unit in order to save money since he was planning to sell the house.
Most outside air conditioner units have a 5 or 10 year manufacturer warranty. An air conditioner repair service can look up the serial numbers and tell you when the unit was manufactured and whether or not it is still under manufacturer warranty. If it is under warranty, you should only have to cover the labor costs usually.
Regardless of what it is, an air conditioner repair service should be able to identify the problem rather easily so I would call them before assuming anything.
