Measure whether the G terminal is receiving a fan signal by testing voltage between the G and C terminals immediately when the thermostat calls for fan operation.

First, confirm power is off at the furnace or air handler before connecting or moving any wires to avoid short circuits or injury; then remove the thermostat cover to access the wiring and note the color and labeling of each conductor, since older or nonstandard installations may use different colors. Next, restore power and set the thermostat fan mode to on so the control should be requesting the fan; if the thermostat has a fan-only button, engage it, otherwise set the system to fan on or perform a fan demand through the thermostat interface. Use a digital multimeter set to measure alternating current voltage in the low-voltage range specific for heating, ventilation, and air conditioning controls, or if you have a multimeter with a selectable range choose the range that covers twenty four to twenty eight volts; connect the meter black lead to the common terminal labeled C and the red lead to the G terminal, making sure the meter leads contact the metal at the terminal screws or exposed wire conductors securely. Read the meter while the thermostat is actively requesting fan operation: a steady reading of approximately twenty four to twenty eight volts indicates the thermostat and wiring are supplying the call for fan and the issue lies in the furnace control board or fan motor circuit; a zero or near zero voltage reading suggests the thermostat is not outputting a fan signal or the conductor is open or shorted. If readings are intermittent or noisy, wiggle the wire at the thermostat and at the control board while observing the meter to locate a loose connection, and verify that the thermostat is properly configured for a conventional fan control rather than communicating with a proprietary control bus. Finally, if the thermostat uses batteries or is a communicating smart thermostat that controls the fan digitally, consult the thermostat manual to confirm whether it supplies physical twenty four volt outputs for the G terminal or if a separate fan relay is required; when in doubt, test continuity from thermostat G to control board G with power off to confirm the conductor is intact.

Common mistakes include testing with power off, which yields no voltage and leads to false negatives; always test with the thermostat calling for fan and power on for accurate results. Another error is measuring between the wrong terminals or using the wrong meter setting, which can damage the meter or give incorrect readings; always connect black lead to common and red lead to G and use the correct low-voltage AC range. A third issue is assuming a modern smart thermostat provides direct twenty four volt outputs when it actually uses digital signaling or an internal relay; if voltage is absent but the thermostat indicates a fan call, consult the device documentation and consider using an external twenty four volt relay.

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