Bosch EasyControl displays error C9 and room temperature readings are clearly incorrect (e.g., showing 5–10°C off actual).

Safety first: Do not open the EasyControl casing; internal components are not user-serviceable.

• 1. Verify with external thermometer: Place a reliable room thermometer near the EasyControl and compare readings over 15–20 minutes.
• 2. Check installation location: Ensure the EasyControl is not above a radiator, in direct sunlight, behind curtains, or on an external wall subject to draughts. If it is, consider relocating using a new Bosch EasyControl wall plate and low-voltage cable.
• 3. Inspect wall cavity: Remove the EasyControl from the wall plate (power off) and check for warm air from cavities, pipes, or cables behind the unit. Seal large gaps with non-combustible filler to prevent air movement affecting the sensor.
• 4. Offset (if available): In the installer menu, apply a small temperature offset to compensate for minor location effects, but do not use this to mask a clearly faulty sensor.

Professional help: If the sensor reading remains significantly inaccurate in a good location, the internal NTC sensor or PCB is faulty and the EasyControl should be replaced.