Vent, Valve & Volt
AC won't turn on, or the outdoor unit isn't running

You can fix this

Likely cause: the thermostat isn't calling for cooling

If the thermostat is on Off or Heat, or set warmer than the room, the AC has no reason to start.

Why this happens

Lennox lists the thermostat among the first things to check when an outdoor unit won't turn on, and suggests setting it at least 3 to 5 degrees below room temperature. A setting only a degree or two below the room may not trigger cooling right away.

How to fix it

  1. Set the mode to Cool and the fan to Auto.
  2. Set the temperature at least 3 to 5 degrees below the current room temperature.
  3. If the screen says Cool On but nothing starts, give it 5 minutes. Honeywell's support page suggests waiting that long when the display shows Cool On or Heat On.
  4. Listen at the outdoor unit. If it still doesn't start, go back and check the breakers.

If this does not fix it, tell the HVAC technician

  • What the thermostat was set to.
  • Whether the screen showed Cool On and how long you waited.
  • The thermostat brand and model.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Lennox: Outside AC unit not turning on
  2. Honeywell Store: Honeywell thermostat not working

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