Thermostat calls for cooling but the outdoor AC unit does not start; indoor blower may run.

Safety first: Turn off power to both the indoor and outdoor units at their breakers before inspecting wiring.

• Step 1 – Confirm cooling mode: Ensure the thermostat is set to Cool and the setpoint is below room temperature.
• Step 2 – Check Y wiring: Verify the Y wire is secure at the thermostat wall plate and at the furnace/air handler control board.
• Step 3 – Check outdoor disconnect: Ensure the disconnect switch near the outdoor unit is inserted and in the ON position.
• Step 4 – Test with jumper: With power on, briefly jumper R to Y at the furnace control board. If the outdoor unit starts, the thermostat or wiring is at fault.
• Step 5 – Continuity test: With power off and Y disconnected at both ends, test continuity of the Y conductor. Replace or repurpose a spare conductor if open.
• Step 6 – Replace thermostat: If wiring and outdoor unit are good, the Y relay on the Geo thermostat may be defective. Replace the thermostat head unit.