Low
4-A1 on Worcester Bosch Greenstar 2000

Your boiler is currently running a programmed air removal sequence to clear bubbles from the heating pipes, which is a normal setup process rather than a mechanical failure.

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.

What the manual says

Manufacturer manual

Venting function: 0: switched off, 1: automatic, 2: permanently switched off. During ventilation, the CO symbol and flow temperature appear alternately in the display.

From this model's manual
Verified against the Worcester Bosch Greenstar 27cdi 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.

DIY fix possible

Wait approximately 10 to 15 minutes for the automated venting cycle to complete naturally.Check that your radiator valves are open to help the air circulate out of the system.If the code persists, restart the boiler by holding the reset button for 3 seconds.

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