the fan won't turn off and just runs constantly no matter what i set it to.

Diagnosis

The constant operation of the fan on a Honeywell Home T4 Pro usually stems from one of three root causes: a software configuration error, a physical wiring short, or a failed relay on the HVAC control board. In most cases, the thermostat is sending a constant 24V signal to the 'G' terminal, which tells the blower motor to run regardless of whether the heating or cooling cycle is active. This often happens if the "Fan Control" setting is accidentally toggled to "On" instead of "Auto," or if a wire has slipped out of its terminal and is bridging a connection. To confirm the source of the problem, you need to determine if the signal is coming from the thermostat or the furnace/air handler. Remove the T4 Pro faceplate from the wall. If the fan stops immediately upon removing the faceplate, the issue is likely a setting or a faulty relay inside the thermostat. If the fan continues to run even with the thermostat removed from the wall, you have a short circuit in the wiring between the wall and the unit, or a stuck contactor/relay on the HVAC control board itself.

How to Fix It

Follow these steps to isolate and resolve the "constant run" issue, starting with the most common software fix before moving to hardware inspection.

1. Check the Fan Mode: On the main screen of the T4 Pro, look at the fan setting. If it says "ON," the fan will run 24/7. Press the "Fan" button repeatedly until the display reads "AUTO." In Auto mode, the fan only runs when the system is actively heating or cooling.
2. Perform a Power Cycle: Turn off the circuit breaker for your HVAC system for exactly 60 seconds. This resets the control board and clears any "stuck" logic states in the T4 Pro's internal relay.
3. Inspect the G-Wire Connection: Pull the thermostat faceplate off the wall. Locate the wire inserted into the 'G' terminal. Ensure the copper wire is stripped exactly 3/8 of an inch and is fully inserted into the terminal. Ensure no stray copper strands are touching the 'R' (power) or 'Y' (cooling) terminals.
4. Test the G-Wire: With the power still off, gently pull the 'G' wire out of the terminal. Turn the power back on. If the fan stops, the wire was likely shorting or the thermostat was sending a false signal. If the fan still runs with the 'G' wire disconnected, the problem is located at the furnace/air handler control board.
5. Verify the Jumper Settings: If you have a heat pump system, ensure the R and Rc terminals are correctly jumped or configured according to your specific system voltage requirements, as an incorrect power bridge can sometimes cause erratic fan behavior.

If That Didn't Work

• Check for a Stuck Fan Relay: On your furnace or air handler's main control board, locate the fan relay. If the relay is physically stuck in the closed position, the fan will run even if the thermostat is disconnected. You can test this by carefully disconnecting the 'G' wire directly from the furnace control board. If the fan keeps spinning, the control board is faulty and needs replacement.
• Inspect for "Fan-On" Delay: Some HVAC systems have a built-in "fan-off delay" that keeps the blower running for 60-120 seconds after a cycle ends to clear the heat exchanger. Wait at least 5 full minutes to ensure you aren't simply seeing a standard system delay.
• Verify C-Wire Voltage: Use a multimeter to check the voltage between the 'R' and 'C' wires. If you are seeing fluctuations or low voltage (below 24VAC), the T4 Pro may experience "ghost" switching, where the internal relays flicker and trigger the fan.
• Replace the Thermostat: If the fan runs only when the T4 Pro is attached and the settings are on "Auto," the internal 24V relay for the fan circuit has likely failed in the "closed" position. This is a hardware failure and requires a full replacement of the thermostat unit.

Heads Up

• Avoid Over-stripping: Never leave more than 1/4 inch of exposed copper outside the terminal block; this is the primary cause of shorts that lead to constant fan operation.
• Voltage Warning: While the system uses 24V AC, which is generally safe, always shut off the main breaker before touching wires to avoid blowing a fuse on the furnace control board.
• Filter Maintenance: A clogged air filter can cause the system to overheat, triggering a "limit switch" that forces the fan to run constantly to prevent the heat exchanger from cracking. Check your filter immediately.
• C-Wire Importance: Ensure you are using a Common (C) wire for power. Using "power-stealing" methods on the T4 Pro can cause the fan relay to chatter or stay engaged.