Thermostat shows E12 and schedules cannot be saved or edited reliably.

Safety first: For battery thermostats, remove batteries before opening. For mains units, isolate power.

• Backup settings: If using Wiser, ensure schedules and settings are synced to the app/cloud.
• Factory reset: Perform a full factory reset of the thermostat and, if applicable, the HubR. Reconfigure schedules from scratch.
• Stable power: Ensure the thermostat has stable power (fresh alkaline batteries or correct mains supply) to avoid further write interruptions.
• Replace unit: If E12 reappears after reset and reconfiguration, the internal flash memory is likely degraded. Replace the thermostat front module or complete unit.