Wiser Room Thermostat shows E09 and the app indicates an internal error; device may reboot repeatedly.

Safety: No mains work required.

• Battery reset: Remove batteries for at least 1 minute, then reinstall new batteries.
• Check for physical damage: Inspect the thermostat for cracks, signs of impact, or water damage. Any such damage can cause internal self-test failures.
• Attempt factory reset: If possible, perform a factory reset as per the manual to clear configuration data.

If E09 persists, the thermostat has an internal hardware fault that is not field-repairable. Replace the thermostat.