To isolate a problem to a specific thermostat wire, remove ambiguity by testing each conductor at both ends while the system is powered and the thermostat calls for operation.

Start by identifying and labeling every conductor at the thermostat and at the furnace or air handler control board so you never confuse wires during testing. With the system powered, remove the green fan wire at the control board and at the thermostat terminal block to isolate that conductor; then measure voltage between the common and the wire in question to see if it is carrying the expected signal when the thermostat commands the fan. If you see no voltage at the control board but voltage at the thermostat end, the fault is in the cable run and you can either replace the cable or reterminate both ends after cleaning and tightening contacts. If there is voltage at the board and none at the thermostat, check for continuity and shorts to other conductors by using a multimeter set to continuity or low resistance, and inspect the cable sheath for damage where it passes through walls, joists, or near sharp edges. When you have an unused spare conductor, temporarily move the thermostat connection to that spare wire at both the terminal block and the thermostat end, then command the function to confirm operation through the alternate conductor. Keep safety in mind: avoid shorting power terminals together, work with one hand when possible near live circuits, and use insulated tools; if you are uncertain about mains level connections, call a qualified technician.

Common mistakes are assuming a dead function means a device failure instead of a wiring issue, which you can avoid by verifying voltage and continuity at both ends before replacing equipment. Another frequent error is swapping wires at only one end, so always move the conductor at both the thermostat and the control board when testing with a spare wire to ensure you have complete isolation. A third issue is poor or corroded terminations that intermittently fail; clean and tighten terminals or cut back and re-strip the conductor to create a fresh contact and that will often restore reliable operation.

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