E03 displayed and thermostat cannot read floor temperature in underfloor heating mode.

Safety first: Isolate power to the heating circuit and thermostat at the main breaker before touching wiring.

• Step 1 – Verify configuration: Enter the installer/advanced menu (refer to Atlantic manual) and confirm that Floor sensor is enabled if a floor probe is used, and that the correct sensor type (e.g. 10k NTC) is selected.
• Step 2 – Check terminals: Remove the thermostat from the wall base. Locate the floor sensor terminals (often marked FS or NTC). Ensure both wires are firmly clamped and not broken or corroded.
• Step 3 – Inspect cable route: Follow the visible part of the floor sensor cable from the thermostat to the floor entry point. Look for pinched, cut, or crushed sections, especially near junction boxes or where tiles were installed.
• Step 4 – Measure resistance: With the sensor disconnected and power off, measure resistance across the floor sensor leads using a multimeter. Compare to Atlantic’s specified value at room temperature (commonly 10 kΩ at 25°C). Infinite resistance indicates an open circuit; near‑zero ohms indicates a short.
• Step 5 – Replace floor sensor: If the sensor is faulty and the cable is in a conduit, pull out the old probe and feed in a new Atlantic‑compatible floor sensor. If the cable is embedded without conduit and cannot be replaced, switch the thermostat to air sensor only mode and install a new thermostat in a better location, or consult a professional for alternative solutions.
• Step 6 – Clear error: After repair, reconnect the sensor, restore power, and reset the thermostat. E03 should clear automatically once a valid resistance is detected.