DIAGNOSIS The thermostat is not detecting voltage on the Y1 wire, which controls the cooling signal. The wire may be disconnected or there is an issue at the HVAC equipment.
FIX Turn off HVAC power. Remove the thermostat from the wall plate. Verify the Y1 wire is securely inserted in the Y1 terminal on the wall plate. At your air conditioner's outdoor unit, check that the low-voltage wire from the Y terminal is connected to the contactor. Restore power.
IF THAT DIDN'T WORK The issue may be a blown fuse on the HVAC control board. Locate the 3-5 amp automotive-style fuse on the board and check it for continuity. Replace if blown.
HEADS UP The 'no power to y1' message can also appear if you have a heat pump system configured as a conventional system in the Sensi's installation settings. Verify your equipment type in the installer menu.