Thermostat clicks or relays chatter rapidly when calling for heat or cool

Safety: Turn OFF power to the indoor unit before tightening connections or inspecting wiring.

• Tighten low-voltage terminals: At the indoor control board and thermostat, tighten all R, C, Y, W, G terminals.
• Inspect wiring: Look for thermostat wires that may be vibrating against metal or moving parts when the blower starts, causing intermittent shorts.
• Measure voltage: With a multimeter, monitor R–C voltage at the thermostat while the system calls for heat or cool. Large fluctuations indicate transformer or wiring issues that must be corrected.
• Replace transformer or thermostat: If voltage is stable but relays still chatter, the thermostat's internal relays may be failing. Replace the thermostat. If voltage is unstable, replace the transformer or repair wiring as needed.