If your fan runs all the time you can check whether the thermostat is sending a continuous fan signal.

Start by turning off power to the heating and cooling system at the breaker to avoid shorts. Remove the thermostat faceplate so the control board remains connected but the thermostat electronics are isolated from the control wiring; if the fan stops when the faceplate is removed the thermostat is the likely source. To confirm with a meter restore power, set a multimeter to measure twenty seven volts direct current and probe between the green fan lead marked G and the common terminal marked C on the furnace or air handler control board while the fan should be off; reading approximately twenty seven volts indicates the thermostat is calling for the fan. If you cannot access the control board safely or the system uses batteries power the stat first then test at the thermostat base between G and C for the same voltage reading. When the thermostat is actively commanding the fan but you are not calling for cooling or fan mode, check thermostat configuration for fan options such as fan on auto versus on or any circulation schedules and disable them. If removing the thermostat face stops the call and configuration is correct replace the thermostat because internal shorts or a stuck relay in the stat are common failure modes.

Three frequent issues are false fan calls due to thermostat internal faults, wiring short between the green wire and a hot terminal, and control board or relay failures at the air handler. If the thermostat is faulty remove power and test between G and C at the control board with the stat faceplate removed and replace the thermostat if voltage disappears when removed. If wiring shorts are suspected visually inspect the G conductor along its run for damage and disconnect at both ends to isolate and test continuity; repair or replace the wire if there is unwanted continuity to other conductors. If the control board or fan relay is sticking diagnose by confirming the thermostat is not calling and then observe whether the relay shows welded contacts or the board has burned traces, and replace the relay or control board if mechanical or electrical damage is present.

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