Diagnosis
The fact that your A/C engages when you jump the wires at the furnace confirms that your HVAC equipment (compressor, condenser, and blower) is functioning correctly, and the issue is isolated to the control circuit between the furnace and the Sensi Touch 2. This typically indicates a "break" in the communication loop. Specifically, the thermostat is failing to bridge the 24VAC power from the R (Red) wire to the Y (Yellow) cooling wire. This failure is usually caused by one of three things: a software-level configuration error where the thermostat doesn't "know" it has a cooling system attached, a physical connection failure at the terminal block (such as a loose wire or a blown fuse on the furnace control board), or a failed internal relay within the Sensi Touch 2 hardware. If the thermostat screen is powered on but the A/C won't kick in, the C-wire is providing power, but the switching logic for the Y-terminal is not engaging.
How to Fix It
Follow these steps to isolate and resolve the connection failure:
- Verify Equipment Configuration: Tap the Menu icon on the home screen. Navigate to Settings > Installation > Equipment. Ensure that "Cooling" is toggled to "Yes." If you have a standard A/C, ensure it is set to "Electric" or "Gas/Electric" and not "Heat Pump" unless you specifically have one. If the setting was "No," the thermostat will never energize the Y terminal regardless of the temperature setting.
- Check the Cooling Delay: The Sensi Touch 2 has a built-in compressor protection delay (usually 5 minutes). Lower the target temperature to 5 degrees below current room temperature and wait exactly 6 minutes to ensure you aren't simply seeing the safety delay in action.
- Inspect Terminal Seating: Power down the HVAC system at the breaker. Pull the Sensi Touch 2 faceplate off the wall base. Inspect the Y (Yellow) wire. Ensure the copper is stripped to exactly 1/4 inch and is inserted fully into the terminal. Tug gently on the wire; if it slides out, re-strip the wire and tighten the terminal screw firmly.
- Test for Continuity: With the thermostat removed from the wall, use a multimeter set to Ohms. Set the thermostat to "Cool" and lower the temp. Check for continuity across the Y and R terminals on the back of the device. If the device is calling for cool but there is no continuity, the internal relay has failed.
If That Didn't Work
- Check the Furnace Fuse: Look at the control board inside your furnace/air handler. There is typically a purple or orange 3-amp or 5-amp automotive-style fuse. If this fuse is blown, the thermostat may still power up via a C-wire, but it cannot send the signal to the Y-terminal. Replace the fuse and check for a short in the yellow wire.
- Verify the G-Wire Connection: If your indoor blower isn't starting, the A/C won't trigger. Ensure the G (Green) wire is securely connected. If you have "Fan Auto" selected, the thermostat manages the G wire; try switching to "Fan On" to see if the blower starts independently.
- Perform a Factory Reset: Go to Menu > Settings > Reset. Perform a full factory reset to clear any corrupted firmware settings that might be preventing the cooling relay from firing. You will need to re-configure your Wi-Fi and equipment settings after this.
- Test with a Dummy Thermostat: If you have a basic single-stage mercury or digital thermostat, wire it in temporarily. If the A/C starts immediately, the Sensi Touch 2 internal circuitry is defective and requires replacement.
Heads Up
- Avoid "Short Cycling": Never rapidly toggle the cooling on and off while troubleshooting. This can cause a high-pressure trip in your outdoor condenser or damage the compressor.
- Voltage Warning: The Sensi Touch 2 is designed for 24VAC systems. Never connect it to 120V or 240V line voltage, as this will instantly fry the logic board and void the warranty.
- C-Wire Importance: While some thermostats claim to work without a C-wire (power stealing), the Sensi Touch 2 performs most reliably with a dedicated 24V Common wire to prevent intermittent power drops that can reset your configuration.