High
20 on Viessmann Vitodens 300-W

Your boiler has lost its connection to the temperature sensor that monitors the main water flow, likely due to a faulty electrical component or sensor.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Controls without flow temperature sensor (low loss header): System flow temperature sensor shorted out

From this model's manual
Verified against the Viessmann Vitodens 200 W installation & service manual, page 92.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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