Cool icon flashes, but the outdoor AC unit does not start.

Safety first: Turn off power to both the air handler/furnace and the outdoor condenser at their breakers before working on any wiring. Outdoor units contain high voltage.

Steps to troubleshoot:

• Check thermostat Y wiring: Remove the thermostat and verify that the Y (or Y1) wire is secure in the Y terminal.
• Inspect air handler/furnace board: With power off, confirm that the Y wire from the thermostat is firmly attached to the Y terminal on the control board.
• Check safety switches: Look for a condensate float switch in the drain line or pan. If tripped (due to a clogged drain), it may open the Y circuit. Clear the drain and reset the switch.
• Bypass test (advanced): With power on and safety switches confirmed, briefly jumper R to Y at the furnace/air handler control board. If the outdoor unit starts, the issue is between the thermostat and the board (wiring or thermostat output).

When to replace: If the system responds to an R-Y jumper but not to the thermostat, the Y output relay in the thermostat is likely faulty, and the thermostat should be replaced.