Blower fan will not turn off and you need a clear step by step diagnostic order to find the cause.

Start by cutting power to the entire system at the breaker or switch and lock it out if possible so you can work safely. Remove the access cover to the air handler and visually inspect for obvious damage, loose wires, or burnt components while power is off. Restore power briefly only when you are ready to measure voltage and use a true root mean square multimeter set to the proper alternating current range for the control circuit; check for G terminal voltage at the control board with the thermostat calling and not calling to establish baseline behavior. Remove the thermostat faceplate to expose its wiring and measure between the G conductor and common or ground while cycling the fan switch; if the thermostat is healthy you should see approximately twenty four volts alternating current when the fan is commanded on and nothing when off. Next, pull the G conductor at both ends, first at the thermostat and then at the control board, to isolate wiring from device control and retest voltage at the board with the wire disconnected; this determines whether the wiring is shorted to a constant source. Inspect the G conductor along its run for pinched insulation, connections at junction boxes, and any contact with transformer or line voltage wires; if the control board output still shows twenty four volts with all low voltage wires removed the board is likely driving the fan and needs replacement.

The first common mistake is testing while leaving multiple wires connected which can cause misleading voltage readings; resolve this by removing the G conductor and all other low voltage leads before rechecking the board output. The second frequent issue is assuming the thermostat is bad without verifying the control board output; pull the G wire at both ends and measure at each point to separate a thermostat fault from a wiring or board fault. The third error is not checking for a stuck fan relay or shorted relay contacts on the board which can keep the fan powered even with no control signal; visually inspect for welded contacts, test the relay coil resistance with power off, and replace the relay or control board if the coil is open or contacts are fused.

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