Thermostat shows E18 and indicates firmware update failure; device may be partially functional.

Safety first: Do not repeatedly power-cycle the thermostat during an update unless instructed by Danfoss, as this can brick the device.

Steps to diagnose and fix:

• 1. Ensure stable power and network: Confirm that the thermostat has a reliable power supply and strong Wi‑Fi signal before retrying the update.
• 2. Retry update via app: Use the Danfoss app or web interface to restart the firmware update. Follow on-screen instructions and do not interrupt the process.
• 3. Use official firmware only: Ensure that the firmware source is the official Danfoss server or service tool. Do not attempt to load third-party firmware.
• 4. Factory reset (if allowed): Some models allow a recovery reset that restores factory firmware. Follow the specific Danfoss recovery procedure for your thermostat.
• 5. Replace thermostat if unrecoverable: If the device remains stuck in E18 or boot loop after recovery attempts, the internal flash or bootloader may be damaged. Replace the thermostat.

When to call a professional: For in-warranty devices or complex recovery procedures, contact Danfoss technical support.