Thermostat calls for cooling but AC does not start

Safety first: Turn off power to the furnace/air handler and outdoor condenser at the breakers before working on wiring.

Steps to troubleshoot:

• 1. Confirm call for cool: Set the thermostat to Cool and lower the setpoint. Look for 'Cool On' indicator.
• 2. Inspect Y wiring at thermostat: Remove the thermostat and ensure the Y (or Y1) wire is firmly connected.
• 3. Inspect Y wiring at furnace: At the furnace/air handler control board, verify the Y conductor is attached to the Y terminal and the cable to the outdoor unit is secure.
• 4. Listen for outdoor unit: When calling for cool, listen for a click at the outdoor unit. No sound may indicate a control issue.
• 5. Jumper test (advanced): With power on and caution, briefly jumper R to Y at the furnace board. If the outdoor unit starts, the thermostat or its wiring is suspect.

When to call a professional: If the outdoor unit does not start even with R-Y jumpered, a technician should check the contactor, capacitor, and high-voltage circuits.