the fan won't turn on when it's supposed to, either auto or when i manually set it to on.

Diagnosis

The Honeywell Home RTH6580WF relies on a 24VAC signal sent from the G terminal to the furnace or air handler's fan relay. When the fan fails to engage in both "Auto" and "On" modes, it indicates a complete break in the communication chain. This is typically caused by a loose wire at the G terminal, a blown 3-amp or 5-amp fuse on the furnace control board, or a failed fan relay. Because the "On" setting bypasses the heating/cooling logic and sends a direct call for the fan, the fact that it fails here suggests the problem is electrical rather than a software setting. To confirm the root cause, you must determine if the thermostat is actually sending the signal. When you switch the fan to "On," the RTH6580WF closes a switch between the R (power) and G (fan) wires. If the thermostat is clicking but the fan isn't spinning, the issue is downstream in the wiring or the HVAC unit. If there is no click and no response, the thermostat's internal relay may have failed, or the common wire (C) is not providing stable power to the WiFi module, causing erratic behavior in the control relays.

How to Fix It

Follow these steps to isolate and resolve the fan failure:

1. Power down your HVAC system at the breaker or the furnace power switch to prevent shorting the control board.
2. Gently pull the RTH6580WF faceplate off the wall base. Inspect the G terminal. Ensure the green wire (standard for fan) is stripped about 1/3 inch and fully inserted into the terminal. Tug the wire lightly; if it slides out, re-insert it and tighten the terminal screw or press the tab firmly.
3. Locate your furnace or air handler. Open the main panel and find the control board. Locate the G terminal on the board and ensure the wire is securely fastened there as well.
4. Check the control board for a small automotive-style blade fuse (usually 3A or 5A). If the filament inside the fuse is broken, replace it with an identical amperage fuse. A blown fuse will kill all signals to the fan regardless of thermostat settings.
5. Perform a "Jump Test" to bypass the thermostat: With the power off, remove the G wire from the thermostat base and the R wire. Use a small piece of insulated jumper wire to connect the R terminal directly to the G terminal. Restore power. If the fan starts immediately, the thermostat is faulty. If the fan still does not start, the issue is a failed blower motor or capacitor.
6. Once the hardware is verified, reset the thermostat configuration. Press and hold the FAN button for 5 seconds to ensure the fan control is set to "Thermostat" and not "Equipment," which allows the furnace to decide when the fan runs based on temperature.

If That Didn't Work

• Check the Blower Capacitor: If you hear the fan motor humming but the blades aren't spinning, the start capacitor is likely dead. This is a cylindrical component on the blower motor. If it looks bulged or leaked, it must be replaced with one matching the exact microfarad (µF) rating.
• Inspect the Limit Switch: In "Auto" mode, the fan is often triggered by a limit switch that detects when the heat exchanger is hot. If this switch is fouled with dust or has failed, the fan won't trigger during a heat call. Clean the sensor with compressed air.
• Verify C-Wire Voltage: Use a multimeter to measure voltage between the R and C terminals. You should see approximately 24VAC. If the voltage is dipping below 22VAC, the WiFi radio in the RTH6580WF can cause "brown-outs" in the switching relays, preventing the G signal from completing the circuit.
• Factory Reset the Unit: If the jump test worked but the thermostat won't trigger the fan, perform a hard reset. Navigate to the settings menu, select "Reset," and confirm. This clears any corrupted firmware glitches that may be blocking the G-terminal output.

Heads Up

• Never touch the blower motor capacitors with your bare hands, even with the power off; they can hold a lethal electrical charge. Discharge them using a resistor or an insulated screwdriver.
• Ensure you are using a 24V system. Connecting this thermostat to a 120V or 240V line will instantly fry the internal circuitry and void the warranty.
• Avoid over-tightening the terminal screws on the baseplate, as this can crack the plastic housing or shear the small gauge thermostat wire.
• If you find a blown fuse on the control board, do not simply replace it and turn it back on. A fuse blows for a reason—usually a pinched wire rubbing against a sharp metal edge of the chassis. Inspect the wire loom for abrasions.