The cooling side of your system makes water as it works. That water drains through a condensate line, usually a small plastic pipe, into a floor drain, a utility sink, or an outdoor outlet. In the spring, before the first long cooling run, check that the line runs clear. Some of this work is yours, and some of it belongs to a licensed hand. The split is easy to draw once you know where the pipe starts and where it ends.
Find the parts first. The condensate outlet sits on the indoor air handler or the furnace section of the system, often in a drip pan under the indoor coil. Follow the pipe from the outlet to where it ends. Look for a float switch, a small device in the pan or on the outlet that shuts the system off if water rises. Note where each part sits before you touch anything.
Look before you open anything. Check for standing water in the pan, slime or algae in the pipe, a damp ceiling or wall below the air handler, and a float switch that is stuck or cracked. Write down what you see. These notes are useful to whoever does the work.
What you can do. Turn the system off at the thermostat and at the switch near the unit. Remove the pan's access cover if it is designed to come off. Clear the outlet end of the pipe with a flexible brush or a wet vacuum if the opening is reachable. If the line has a labeled cleanout cap, you may flush it with clean water. Wipe the pan dry, check that the float switch moves freely, and restore power. Run a short test and watch the pan through the next run.
What calls for a licensed hand. Bring in a licensed technician if the line runs through a wall or ceiling and you cannot reach the clog, if the pipe has no cleanout, if the clog returns soon after clearing, if the pan rusts through or cracks, or if the float switch trips again after you have cleared the line. Any work on wiring at the switch belongs to a licensed electrician or technician. A licensed hand may also need to reroute the pipe or confirm that the pan is set to drain.
Compare the two routes. Weigh the time, the access, the tools you have, and the cost of water damage if the job goes wrong. Ask the technician where the line terminates, whether it needs a trap, whether the pan is set for drainage, and whether they check the float switch during a live run. Compare their answers with what you found in the house file.
Note the spring check in the house file, even if the line ran clear. Pair it with the rest of the cooling-side check. The Central Air Conditioner page on typical service life and when to plan covers the cooling side, and if your system is a heat pump, the Heat Pump page on typical service life and when to plan covers the same ground.