Thermostat displays E99 and will not operate HVAC equipment

Safety first: Turn off power to the HVAC system at the breaker before removing the thermostat from the wall.

Steps to troubleshoot:

• 1. Power cycle thermostat: Remove the thermostat from the wall plate for 30 seconds, then reattach. Also cycle the HVAC breaker off and on.
• 2. Check for visible damage: Inspect the back of the thermostat for burn marks, swollen components, or corrosion. If present, the thermostat must be replaced.
• 3. Verify correct wiring: Confirm that R, C, W, Y, G, and other terminals are wired according to the Resideo installation guide for your system type. Incorrect wiring can damage the control board.
• 4. Factory reset: Perform a factory reset from the thermostat's advanced settings menu (refer to your model's manual). Reconfigure system type and Wi‑Fi.
• 5. Test with a basic thermostat: If available, temporarily install a simple non-smart thermostat to verify that the HVAC equipment itself operates normally.
• 6. Replace thermostat: If E99 persists and the HVAC system works with a different thermostat, replace the Resideo smart thermostat.

When to call a professional: If you suspect surge damage or incorrect system configuration, have a licensed HVAC technician install and configure the replacement thermostat.