Thermostat displays A2 and cannot read room temperature; heating control becomes erratic or fixed to a default value.

Safety first: Turn off power to the thermostat circuit at the breaker or remove batteries if it is battery‑powered.

• Check mounting location: Ensure the thermostat is not installed above a radiator, in direct sunlight, or near a heat source that could overheat the internal sensor.
• Inspect for moisture or contamination: Remove the thermostat from its wall plate and look for signs of condensation, water damage, or dust buildup on vents. Allow the unit to dry completely if moisture is present.
• Soft reset: If the thermostat has a reset function in the menu, perform a factory reset (note your schedules and settings first). For battery models, remove batteries for 1–2 minutes and reinstall.
• Check temperature reading: After power is restored, compare the displayed room temperature to an accurate thermometer. If the reading is missing or clearly wrong and A2 persists, the internal sensor is likely defective.
• Replace thermostat: The room sensor is typically integrated into the thermostat PCB and not user‑serviceable. Replace the thermostat with a compatible Viessmann smart thermostat model.