The boiler is not sensing proper water circulation, so it has shut down to protect itself.
Most common on: Greenstar i, CDi Classic
A1 means the boiler's dry-fire protection has activated: it is not detecting enough water movement through the heat exchanger. Firing without circulation would let the heat exchanger overheat, so the boiler stops.
It is usually a circulation problem — a stuck pump, an airlock or low pressure — rather than a fault with the burner itself.
If pressure is below 1 bar, top up using the filling loop to around 1.0–1.5 bar (cold) and reset.
With the heating calling for heat, feel whether the pump is running and warm. A silent, cold pump may be seized or airlocked.
Bleeding radiators and the boiler can clear an airlock, but if the fault returns it needs an engineer to check the pump and sensors.
A failing pump, sensor or blockage needs a heating engineer to diagnose and replace parts.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler is not sensing proper water circulation, so it has shut down to protect itself. A1 means the boiler's dry-fire protection has activated: it is not detecting enough water movement through the heat exchanger. Firing without circulation would let the heat exchanger overheat, so the boiler stops.
You can try a single reset and the safe checks below, but Worcester Bosch A1 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring A1 means the underlying cause has not been fixed — most often low system pressure (below about 1 bar). Resetting repeatedly will not solve it; get the boiler diagnosed properly.
Low water pressure
Overheat — primary temperature too high
Internal / sensor fault
See all Worcester Bosch fault codes.