Something StrangeYou know your unit. If it is making unusual noises, leaving moisture where it shouldn't, or producing funny smells, there is most likely a problem and it is time to call an air conditioning repair technician. A properly functioning air conditioner should be quiet, with no scraping or banging noises. It should be free of leaks with no puddles on the floor or underneath the unit. And any noticeable musty or moldy smell is a sure sign of trouble.
Low FlowAnother reason to call for ac repair is a perceptible decrease in air flow. You could try removing the filter from your unit and cleaning it with a vacuum cleaner and rinsing the filter in the bathtub to remove dirt and grime. If this doesn't improve your air conditioner's performance, it may be a problem with an internal part, which would be something a repair person would have to fix.
No ChillThe most noticeable issue with a malfunctioning air conditioner is that it stops blowing cool air. This will mean a call to the ac repair company. It could be as simple as the system needing to be recharged with a can of freon, but it could also mean that the compressor in your unit is not working. Only a trained, reputable air conditioning repair person can tell you exactly what is wrong.
If you notice that your air conditioner does not seem to be working the way it has in the past, it is time to call for ac repairs. Don't wait until it stops entirely because this will be a long, hot summer.