Most homeowners never see the drain pan under an indoor air handler, and fewer have looked at the small float switch riding in it. That switch is a cheap safety device. When condensate rises to a set height, the float lifts and opens a control circuit, which cuts power to the cooling call before water spills onto a ceiling or floor. The job is simple. The failure modes are not, and both are easy to check with a multimeter.
Some installs use a float in a primary pan, others add a secondary pan with its own switch. Either way, the switch sits in the control path, so it can stop cooling even when nothing is wrong with the compressor. That is why it is worth testing, not just knowing it exists.
What a stuck switch does
Two failures matter. A switch stuck open will kill the cooling call while the drain runs fine, and the complaint sounds like "the air conditioner quit," which sends people to the outdoor unit first. A switch stuck closed, or wired so it cannot open, removes the safety net entirely, and an overflowing pan becomes a water-damage problem. Both show up on a meter before they show up in the ceiling.
Wiring varies by manufacturer, so read the diagram on the air handler or furnace door before you touch anything. Many float switches are wired into the low-voltage control circuit, with the leads landing on two terminals. A secondary pan float on some installs may also drive an alarm relay or a separate indicator, so trace where every lead goes before you assume the switch has only one job.
Test it with the power off
Start at the disconnect and shut off power to the air handler. Confirm the circuit is dead with a voltage check before you open the panel. Then disconnect the float leads, set the meter to continuity, and measure across the switch terminals with the float resting in its normal position. A healthy normally closed switch reads continuity at rest. Lift the float by hand to its trip height and the reading should drop to open. Release it and continuity should return.
If the reading does not change when you lift the float, the switch is stuck, the leads are disconnected, or the float is jammed against the pan wall. Clean the pan of debris and check that the float moves freely on its guide before you decide the switch is bad. If it does need replacing, use a part rated for the same control circuit and label the leads as you remove them.
Check the drain at the same visit
A float switch only protects you if the drain behind it is moving water. Pour a measured amount of water into the pan's drain opening and watch how fast it clears. Slow clearing points to algae or debris in the trap or line, which is where cleaning should start. A pan that fills faster than it drains is the real problem, and the float is only the last line.
Log what you found: the date, the reading at rest, the reading at trip, and whether the drain cleared on its own. Next season you will know whether the switch changed or the drain did.
Where a service visit fits
A pre-season service visit is a good time to ask the technician to show you the switch and the pan. ACHR News, in 6 HVAC Maintenance Topics to Discuss With Homeowners, describes routine visits as a chance to help homeowners decide things "before extreme weather puts their HVAC systems to the test." Asking to see the float test is one of those decisions, and it takes a few minutes. If the switch is stuck, a technician will likely find it on the first pass, but you will know what failed and why.