High
E1 on Worcester Bosch Greenstar 25Si

Your boiler has detected an issue with how it clears exhaust fumes, causing it to stop running for safety.

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

Flue pressure switch failure

From this model's manual
Verified against the Worcester Bosch Greenstar 25si installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the boiler's safety sensors detect that exhaust fumes are not being cleared safely through the outdoor flue pipe. It is typically caused by a blockage in the air pressure tubes, a build-up of debris in the fan, or a mechanical failure in the switch that monitors airflow.

This issue is more frequent during periods of high winds or extreme cold, which can cause pressure fluctuations or ice blockages in the external flue terminal.

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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