the fan on my system won't turn off even when the heat or ac isn't running, and my lux geo says fan is in auto mode.

Diagnosis

This issue is rarely a software glitch within the Lux GEO interface, even if the display indicates "Auto." When the fan continues to run despite the thermostat requesting a stop, it typically signifies a "closed loop" scenario where the fan relay is being energized regardless of the thermostat's state. This happens because the G-wire (the fan control line) is receiving a constant 24VAC signal, or the mechanical relay on the furnace/air handler control board has physically welded shut. To confirm this, you need to isolate the thermostat from the HVAC equipment. If the fan continues to blow after the thermostat is physically removed from the wall plate, the problem is downstream—either a short circuit in the wiring harness between the wall and the unit, or a failure of the fan relay on the integrated furnace control (IFC) board. If the fan stops immediately upon removing the thermostat, the issue is a faulty internal relay within the Lux GEO unit itself, which is failing to break the connection to the G-terminal.

How to Fix It

Follow these steps to isolate and resolve the constant fan operation:

1. Shut off the power to your HVAC system at the main electrical breaker panel to prevent electrical arcs or shorting.
2. Gently pull the Lux GEO thermostat body off its wall mounting plate. Note that the pins are fragile; do not rock the unit side-to-side.
3. Locate the Green wire connected to the G terminal. Using a small precision screwdriver, loosen the terminal screw and pull the G-wire completely out of the connector.
4. Wrap the exposed end of the G-wire with electrical tape to ensure it does not touch any other wires or the metal chassis of the wall box.
5. Restore power at the breaker.
6. Observe the fan. If the fan stops, the thermostat was sending a false signal or has a faulty internal relay. You will need to replace the Lux GEO unit.
7. If the fan continues to run with the G-wire disconnected, the issue is the control board. Locate the HVAC control board inside your furnace or air handler. Find the G-terminal on the board. Carefully disconnect the G-wire from the board side.
8. If the fan stops now, you have a short in the thermostat wiring (the wire is touching another hot wire inside the wall). If the fan keeps running even with the wire removed from the board, the fan relay on the control board is welded shut and the board must be replaced.

If That Didn't Work

• Check for "Fan On" Jumpers: Inspect the control board for any installed jumpers between the R and G terminals. Some installers use a physical jumper wire to force the fan to run during commissioning; if this was accidentally left in place, the fan will ignore the Lux GEO's "Auto" command.
• Inspect the Float Switch: If you have a secondary drain pan with a safety float switch, a partially failed or corroded switch can sometimes trigger a "fail-safe" mode on certain HVAC brands that forces the blower to run to prevent humidity buildup, even if the thermostat says otherwise. Clean the pan and ensure the switch is seated correctly.
• Verify C-Wire Voltage: Use a multimeter to check the voltage between the R (Red) and C (Common) wires. If the Lux GEO is not receiving a steady 24V, it can cause erratic relay behavior. If you are using a "power extender" or "add-a-wire" kit, ensure the connections at the furnace board are tight and not bridging into the G-terminal.
• Test the Blower Motor Capacitor: While rare, a failing run capacitor can cause the motor to behave erratically, though this usually results in a humming sound or a slow start rather than a constant "on" state. Check for bulging or leaking capacitors on the blower motor.

Heads Up

• Warning: Always turn off the breaker before touching the control board. The 120V side of the furnace board can be lethal, and the 24V side can blow the integrated fuse if you accidentally touch the R-wire to the chassis.
• Wire Integrity: When reconnecting wires, ensure no stray copper strands are poking out of the terminals. A single strand of copper touching the neighboring terminal can trigger the fan or the AC compressor.
• Voltage Check: Only use a multimeter set to AC voltage when testing the G-wire. Testing with a DC setting will give you an incorrect reading and may lead to an incorrect diagnosis.
• Cleaning: While the system is powered down, use compressed air to clear dust from the control board. Excessive dust buildup can occasionally cause "phantom" shorts across relay pins.