The float switch.

A float switch sits in the drain pan or spliced into the condensate line. It is a small sealed box with a float in it. When water backs up, the float rises, a contact opens, and the low voltage control circuit is broken. The system shuts down.

That is the whole device. It has one job and it does it by taking your air conditioning away.

The symptom is nothing

This is the part worth knowing. The most common version of this call is not a leak and not a noise. It is a system that quietly stops.

The thermostat still has power. The breaker is on. Nothing smells, nothing is iced, nothing is making a sound. The house just gets warmer and there is no fault to find, because nothing failed. A switch did its job.

If your AC is dead and everything looks normal, go and look at the drain pan before you look at anything else.

Do not bypass it

Jumping a tripped float switch to get cooling back is a real temptation on a hot afternoon, and it is the single worst trade in residential HVAC. You are removing the only thing standing between a plugged drain and drywall. Attic units sit directly over ceilings.

Clear the drain instead. A wet vacuum on the outdoor termination gets a good share of plugs out, and vinegar down the access tee every couple of months through the season mostly prevents the next one.

When the switch itself is the fault

They do fail, usually stuck open, which gives you a system that will not run with a perfectly clear drain. That is a flat price on the list, as is clearing the line.