4-A1 Low

Worcester Bosch 4-A1 Fault Code

Also written as 4A1 — same fault.

The Worcester Bosch 4-A1 fault code means: 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.. It appears on 1 Worcester Bosch model in our database.

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

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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Worcester Bosch models that show the 4-A1 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Worcester Bosch 4-A1 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026