Cold weather tends to expose the condensate side of a system before anything else. The indoor coil pulls moisture out of your house air all through the cooling season, and that water has to leave through a small drain line. When the line is blocked, slow, or sitting in cold air, water backs up into the drain pan, trips a safety switch, or freezes at a spot nobody looks at. This check takes about an hour. You need a multimeter, a flashlight, a shop vacuum, and some patience in the attic or crawlspace.
Trace Every Drain Path First
Most systems have a primary drain and a secondary or emergency drain. The primary usually runs from the pan to a floor drain, a wall pipe, or a condensate pump. The secondary may exit through a wall above a window, or it may be a separate pan with its own float. Follow both with a flashlight. Note where each one ends, whether it has a visible downhill slope, and whether it passes through unheated space. Write the path on paper before you touch anything. A drain you cannot trace is a drain you cannot check.
Check the Trap, the Slope, and the Flow
A condensate trap holds a column of water so the blower does not pull air through the drain line. Find the U-shape, confirm the cleanout cap is seated tight, and look for cracks where the trap meets the pan. The drain line needs a steady run to its outlet. If it sags into a low loop, water sits in the bottom, gathers sludge, and freezes sooner. With the system off, pour a measured cup of clean water into the drain pan and time how long it takes to leave. Slow drainage means partial blockage. Water still standing in the pan after a few minutes means the blockage is serious enough to clear before the next cooling run.
Test the Float Switch With the Meter
Many pans carry a float switch that cuts power to the air handler when water rises. It is the part that keeps a clogged drain from becoming a ceiling problem. Turn off the breaker, lock it out, and disconnect the switch leads. Set the multimeter to continuity or resistance and check the switch with the float at rest. Then lift the float by hand and check again. A switch that does not change state when you lift the float is not telling the control board anything useful. Replace it before you rely on it. Reconnect, restore power, and confirm the system starts normally.
Follow the Outdoor End
If the drain exits through a wall into an outdoor spot, check that end for ice, leaves, and insect nests. A line that discharges onto a slab or into gravel can collect ice at the outlet. A line that runs through an attic or crawlspace needs insulation or a route that stays above freezing. Do not add heat tape unless the product is listed for that application and you have checked the manufacturer's instructions. Mark any stretch of line that runs flat or dips, because those are the spots that freeze first.
Ask the Tech to Show the Numbers
When a contractor visits, ask to see the trap, the slope, the pan, and the float switch. Ask them to record the state of each item on the ticket. A note that says "drain checked" tells you very little. A note that says the trap was sealed, the float changed state when lifted, and a cup of water cleared in a timed interval gives you a baseline for next year.
Referral work runs on the same trust. ACHR News has a piece titled How HVAC Contractors Can Boost Referral Business, and its summary reads "Four marketing experts offer tips for earning more referral business from happy customers." That framing addresses the contractor side of the relationship. For a homeowner, the useful flip side is simple: a customer who can name what was checked, and what the readings were, is better placed to judge whether the next visit is worth the same money, and to recommend a contractor who showed their work.