installed my sensi lite and now my ac fan won’t stop running, even when the system is off.

Diagnosis

The continuous operation of the indoor blower fan on an Emerson Sensi Lite ST26 typically stems from a "Fan Control" conflict between the thermostat and the furnace/air handler. In most modern HVAC systems, the furnace control board is designed to manage the blower speed and timing—specifically the "fan-off delay" that allows the heat exchanger to cool down after a cycle. When the Sensi Lite is configured to control the fan directly, it sends a constant 24V signal to the G terminal, overriding the furnace's internal logic and keeping the fan running indefinitely. This happens most frequently during initial installation when the "Fan Control" setting is defaulted to "Thermostat" rather than "HVAC." You can confirm this by removing the Sensi Lite faceplate from the wall. If the fan stops immediately after the thermostat is disconnected, the issue is a software configuration setting within the ST26. If the fan continues to run even with the thermostat removed, you have a physical short in the G-wire or a stuck relay on the furnace control board.

How to Fix It

To resolve the fan run-on issue, you must reconfigure the internal equipment settings to hand control back to the HVAC system. Follow these exact steps:

1. Wake the Sensi Lite display by touching the screen.
2. Press the Menu icon (three horizontal lines) located in the bottom left or right corner of the home screen.
3. Navigate to Installer Setup. You may be prompted for a passcode; if you haven't set one, the default is usually blank or 0000.
4. Select Equipment from the setup menu.
5. Scroll through the equipment options until you find Fan Control.
6. Change the setting from "Thermostat" to HVAC. This tells the Sensi Lite to let the furnace/air handler determine when to start and stop the blower.
7. Press Done or the back arrow to save the settings and return to the main screen.
8. Wait approximately 5 minutes. The fan may not stop instantly due to the furnace's internal cool-down timer, but it should shut off once that timer expires.

If That Didn't Work

• Check for a G-Wire Jump: Inspect the wiring behind the thermostat. Ensure there isn't a physical jumper wire connecting the R (power) terminal directly to the G (fan) terminal. Some older installations used jumpers to force the fan on; this will override any software setting and keep the fan running 24/7.
• Verify the C-Wire Connection: If you are using a "power stealing" configuration without a C-wire, the ST26 may be drawing enough current through the G-circuit to trick the furnace into thinking the fan is being requested. Ensure the C-wire is securely seated in the C terminal and connected to the 24VAC transformer at the furnace.
• Reset the Furnace Control Board: Power cycle your entire HVAC system. Flip the breaker or switch the furnace power switch to OFF for 60 seconds, then flip it back on. This clears any "stuck" logic on the furnace board that might be ignoring the "stop" command from the thermostat.
• Test for a Stuck Relay: If the fan runs even with the thermostat completely removed from the wall, the fan relay on the furnace control board has likely failed in the "closed" position. This requires a technician to replace the control board or the relay capacitor.

Heads Up

• Avoid "Fan On" Mode: Ensure the system is set to "Auto" and not "On" on the main screen. The "On" setting is designed for circulation and will run the fan regardless of temperature.
• Voltage Warning: Always turn off the breaker before touching the wiring. A short between the R-wire and the chassis can blow the 3-amp or 5-amp fuse on your furnace control board instantly.
• Compatibility Check: The Sensi Lite is designed for single-stage systems. If you have a high-efficiency multi-speed blower, selecting "Thermostat" control can actually damage the blower motor by running it at the wrong speed for too long.