Display shows 'Er4' and thermostat time/date resets frequently.

Safety: No high-voltage work is necessary if the unit is battery powered; isolate mains for wired models.

• Check main power/batteries: Ensure the thermostat has a stable power source and that batteries are not loose or depleted.
• Re-enter time/date: Set the correct time and date, then monitor if they are retained after a short power loss.
• Firmware update: Update to the latest firmware to fix known RTC issues.
• Replace thermostat: If time resets continue and Er4 remains, the RTC or backup circuit is defective. Replace the thermostat.