i'm getting an 'installation error' message on screen after trying to install it myself, won't go away.

Diagnosis

The "Installation Error" message on the Emerson Sensi Touch ST75 is triggered by the device's internal logic board detecting a conflict between the physical wiring connected to the backplate and the software configuration selected during the initial setup wizard. This typically happens when the thermostat detects voltage on a terminal that it believes should be empty, or conversely, when it detects a missing connection for a required terminal (like the C-wire) based on the system type you selected (e.g., Heat Pump vs. Conventional). The ST75 is particularly sensitive to "ghost voltages" or improper grounding. If you have a heat pump system but selected "Conventional" in the setup, or if you have a jumper wire installed between the R and Rc terminals that the software doesn't expect, the system will throw this error to prevent potential damage to the control board. You can confirm this by checking if the error appears immediately upon power-up or only after you attempt to save the system configuration settings in the menu.

How to Fix It

To clear a locked "Installation Error" and correctly calibrate the device, you must perform a hard reset and a precise wiring audit. Follow these exact steps:

1. Power down the HVAC system at the circuit breaker to ensure no voltage is flowing to the thermostat during the physical audit.
2. Remove the ST75 faceplate from the wall bracket. Inspect the wiring. Ensure no copper strands are fraying or touching adjacent terminals. Specifically, check that you do not have a jumper wire installed between Rh and Rc if you are using a single R wire; the ST75 handles this internally.
3. Reattach the faceplate and restore power at the breaker.
4. To clear the error screen, press and hold the Menu button (or the center of the screen if touch-responsive) for exactly 10 seconds. This forces a factory reset and wipes the corrupted installation profile.
5. When the setup wizard reappears, carefully select your system type. If you have an O/B wire, you MUST select "Heat Pump." If you have only R, G, W, and Y wires, select "Conventional."
6. Verify the "C-Wire" setting. The ST75 requires a constant 24VAC power source. If you do not have a C-wire and are using a power extender kit, ensure the kit is wired to the furnace control board exactly as specified, otherwise, the installation error will return the moment the WiFi chip attempts to ping the router.
7. Save the settings and allow the device to run its 30-second self-test.

If That Didn't Work

• Check for O/B Reversal Conflict: If you have a Heat Pump and the error persists, go into the Advanced Settings and toggle the "Reversing Valve" setting from "O" to "B" (or vice versa). Some brands like Rheem or Ruud require the B setting, and a mismatch here can trigger a configuration error.
• Test for a Blown Fuse: Check the 3-amp or 5-amp fuse on your furnace/air handler control board. If the fuse is partially blown or intermittent, the ST75 may receive enough voltage to boot but not enough to complete the installation check, triggering the error.
• Isolate the C-Wire: Use a multimeter to test the voltage between the C terminal and the R terminal. You should see a steady 24V AC. If the reading fluctuates or is below 22V, the thermostat will trigger an installation error because it cannot maintain a stable power rail for the touch screen.
• Verify the Common Wire Connection: Ensure the C-wire is securely fastened at both the thermostat and the furnace. A loose connection at the furnace control board often manifests as a software "Installation Error" because the device detects a voltage drop during the initial power-on self-test.

Heads Up

• Never use a jumper wire between Rh and Rc on the ST75 unless you are powering the system from two completely separate transformers.
• Avoid using "Power Stealing" methods or add-a-wire kits that aren't specifically rated for the ST75's power draw, as the touch screen requires more current than the non-touch Sensi models.
• Always shut off the breaker before touching the wiring; shorting the R wire to the C wire can instantly fry the 24V transformer on your HVAC board.
• Ensure your WiFi signal strength is "Good" or "Excellent" during setup; occasionally, a failed cloud registration during the installation phase can be misreported as a hardware installation error.