Low Warning
CE on Viessmann Vitodens 100-W / 200-W

Your boiler is having trouble communicating with an add-on control or an external smart home accessory, which may affect its ability to run efficiently.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

Control mode - Communication fault-ext. extension

What causes this fault?

This fault occurs when the boiler's main control board loses the digital handshake with an external accessory, such as a smart home interface or an expansion module. The most common reason is a loose connection in the low-voltage wiring or a hardware failure within the add-on component itself. Without this communication link, the boiler can no longer receive instructions on how hard it needs to work to heat your home.

DIY fix possible

Check if any external smart thermostats or controllers are turned on and have battery power.Restart the boiler by turning the power off and back on at the fused spur switch.Ensure any wireless hubs or range extenders nearby are plugged in and working.

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Related fault codes

CDView →CFView →B0View →

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