my sensi keeps showing 'cooling' or 'heating' but the system never actually turns on

Diagnosis

This issue occurs when the Sensi ST55 is successfully triggering a "call" for climate control—indicated by the on-screen status—but the physical electrical circuit is not being completed to the HVAC equipment. In a properly functioning system, the thermostat acts as a switch that connects the 24VAC power from the R wire to the specific call wire (Y for cooling, W for heating). When the screen says "Cooling" but nothing happens, the ST55's internal relay has closed, but the signal is being blocked by a loose wire, a blown fuse on the control board, or a tripped safety switch. To confirm if the issue is the thermostat or the HVAC system, you need to check for the "click." When the ST55 engages, you should hear a distinct mechanical click from the baseplate. If you hear the click but no air flows, the problem is likely downstream in the wiring or the furnace/air handler. If you do not hear a click, the internal relay may be faulty, or the C-wire (common) is not providing consistent power to sustain the relay's hold. You can also confirm this by removing the thermostat faceplate and jumping the R wire to the Y wire (for cooling) using a small piece of insulated copper wire; if the system kicks on, the thermostat is the culprit.

How to Fix It

To resolve this, we will first isolate the hardware using the built-in diagnostic tools and then verify the physical connections at the baseplate.

1. Remove the Sensi faceplate from the wall and inspect the wiring. Ensure that every wire is stripped to exactly 1/4 inch and is fully inserted into the terminal blocks. Tug gently on the R, Y, and W wires to ensure they are clamped tight.
2. Snap the faceplate back on and navigate to the Menu button on the home screen.
3. Select Settings, then select Equipment, and finally choose Test Equipment.
4. Select Cooling. Listen closely to the thermostat. You should hear a click. Wait 5 minutes to account for the compressor's built-in short-cycle delay.
5. If the outdoor unit doesn't start, repeat the process for Heating.
6. If neither works, power down your HVAC system at the breaker. Open the indoor air handler panel and locate the control board. Look for a small 3-amp or 5-amp automotive-style blade fuse (usually purple or amber). If the filament is broken, replace it with an identical amperage fuse.
7. Restore power and re-run the Test Equipment sequence.

If That Didn't Work

• Check the Float Switch: Many AC systems have a condensate overflow switch (a small plastic T-junction on the drain line). If your drain line is clogged, the switch trips and breaks the R-wire connection to prevent flooding. This will allow the Sensi to show "Cooling" but will physically prevent the system from turning on. Clear the drain line with a shop vac to reset the switch.
• Verify the C-Wire Connection: While the ST55 can work without a C-wire in some configurations, "phantom power" issues often cause the relay to fail under load. Ensure the blue or black C-wire is connected both to the Sensi terminal and the transformer at the furnace. If you are using a power extender kit, verify that the wires are seated firmly in the puck.
• Inspect the Contactor: If you hear the indoor fan start but the outdoor condenser remains silent, the 24V signal is reaching the outdoor unit, but the contactor (the heavy-duty relay in the outdoor unit) is burnt out or stuck. This requires a multimeter to check for 24VAC across the contactor coil terminals.
• Reset the HVAC Control Board: Occasionally, the furnace or air handler control board enters a "lockout" mode due to a previous error. Turn the breaker off for a full 60 seconds and then back on to clear the volatile memory of the control board.

Heads Up

• Short-Cycle Protection: Always wait at least 5 minutes between tests. Most modern HVAC systems have a built-in delay to protect the compressor from pressure spikes; if you toggle the Sensi on and off rapidly, the system will ignore the call for 5-10 minutes.
• Wire Gauge Warning: Ensure you are using 18-gauge thermostat wire. Using thinner wire over long distances can cause a voltage drop, meaning the Sensi thinks it's sending a signal, but the voltage is too low to trigger the HVAC relay.
• Avoid "Jumping" with High Voltage: Never apply 120V or 240V power to the Sensi terminals. This is a 24VAC low-voltage device; applying mains power will instantly fry the logic board and void the warranty.
• Battery Maintenance: If you are not using a C-wire, replace the batteries every 6 months. Low batteries can lead to "ghost calls" where the screen indicates a state that the hardware cannot actually execute.