Thermostat display shows F0 and boiler does not respond to temperature changes

Safety first: Turn off power to the boiler and thermostat circuit at the main switch or breaker before removing any covers. Do not work on live wiring.

• Power reset: Turn off the boiler and thermostat power for 2–3 minutes, then turn back on. Sometimes F0 is cleared by a full reboot.
• Check power supply: If the thermostat is wired to the boiler (eBus/OTBus), verify the boiler is powered and not in lockout. If it uses a 230 V adapter, ensure the outlet has stable voltage and the plug is firmly seated.
• Inspect location: Make sure the thermostat is not installed above a radiator, in direct sunlight, or near a heat source that could confuse the internal sensor.
• Check for condensation: If the thermostat is in a humid area, remove it from the wall plate (if clip-on type) and visually inspect the PCB for moisture. Allow it to dry completely before refitting.
• Factory reset: From the thermostat menu, perform a factory reset (consult the Saunier Duval smart thermostat manual for the exact sequence). Reconfigure Wi‑Fi and boiler pairing afterward.

If F0 persists after reset and relocation, the internal sensor or PCB is likely defective and the thermostat should be replaced.