Thermostat clicks but HVAC equipment does not start

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breaker before inspecting wiring or control boards.

Steps to diagnose:

• Listen for relay: The click indicates the thermostat is attempting to energize a call (heat, cool, or fan).
• Check furnace board fuse and LEDs: With power off, inspect the low‑voltage fuse. After restoring power, check for diagnostic LEDs indicating lockout or faults.
• Test with jumper (advanced): At the furnace control board, with power on and door switch engaged, briefly jumper R to the appropriate terminal (W for heat, Y for cool, G for fan). If the equipment starts, the issue is between the thermostat and board.
• Inspect thermostat wiring: Confirm all conductors are intact and firmly connected at both thermostat and furnace ends.

When to replace: If the equipment responds to manual jumpers but not to the thermostat, the internal relay contacts may be burned or open. Replace the thermostat.