Nest thermostat calls for cooling but air conditioner does not start

Safety: Turn off power to both indoor and outdoor units before opening panels.

• Confirm call for cool: Ensure the Nest shows a cooling call (blue ring) and the setpoint is below room temperature.
• Inspect Y1 wiring: Verify the Y1 wire is secure at the Nest base and at the furnace control board Y terminal, and that the outdoor unit control wires are properly connected.
• Fuse inspection: Replace a blown low‑voltage fuse on the furnace control board with the same rating.
• Condenser check: At the outdoor unit, inspect the contactor coil connections and any inline safety switches. Reset any manual reset switches if present.
• Direct jumper test: With Nest removed and power on, jumper R to Y at the furnace control board. If the condenser starts, the issue is with the Nest or thermostat wiring. If not, the condenser contactor, control board, or safety switches may be faulty and require a technician.