You can fix this
Likely cause: a float switch shut the AC off
Water backing up in the drain pan can trip a float switch that cuts power to the AC. Clear the drain and it should restart.
Why this happens
Lennox lists a tripped float switch among the reasons an AC won't turn on, and Honeywell notes a condensate float switch can cut power to the system. Trane lists a clogged drain line as a cause too. The switch is a safety device: it stops the AC so the pan doesn't overflow onto your floor or ceiling.
How to fix it
- Set the thermostat to Off and switch off power to the indoor unit.
- Soak up standing water with towels or a wet/dry vac.
- Clear the drain line with the vacuum and vinegar steps in the AC leaking water troubleshooter.
- Once the pan drains, restore power and set the thermostat to Cool. The float switch should reset as the water goes down.
When to stop. If the pan fills again within weeks, or you can't reach the clog, Lennox says it's time for a professional.
If this does not fix it, tell the HVAC technician
- That you found water in the pan and the AC wouldn't start.
- How you cleared the drain and whether the clog has come back before.
- Any rust in the pan or water damage nearby.
Build the full list: what to tell the HVAC technician.
Start this troubleshooter over
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