Worcester Bosch A1 Fault Code
The A1 fault indicates the PCB cannot confirm the fan is operating correctly, or the flue pressure switch has not triggered as expected. The boiler locks out as a safety measure — it will not fire without confirmed flue clearance.
What is the Worcester Bosch A1 fault code?
The Worcester Bosch A1 fault is triggered when the boiler's control board does not receive the correct signal from the air pressure switch (APS) within the required window after the fan starts. The APS is a small device that confirms flue gases can escape safely before ignition is permitted. If the fan is not running at speed, or if the flue is restricted, the APS cannot register the required pressure differential and the boiler locks out with A1.
What causes the A1 fault?
The fan motor degrades over time and may run intermittently or too slowly to generate sufficient flue draught. This is the most common cause of A1 on Greenstar boilers over 8 years old.
A bird nesting in or near the flue terminal, debris accumulation, or a flue that has come loose and is partially obstructed will restrict airflow. The APS cannot register the required pressure and trips the A1 lockout.
The APS diaphragm can crack or the switch contacts can fail over time. It may show A1 even when the fan is running correctly because the switch cannot detect the pressure change.
On condensing boilers, a blocked condensate trap can fill with water and restrict the flue gas path — effectively mimicking a flue blockage and triggering A1.
A small rubber hose connects the APS to the fan assembly. If this hose cracks, splits, or becomes dislodged, the APS never registers flue pressure regardless of fan speed.
Rarely, the PCB misinterprets fan speed signals and generates a false A1. Diagnosed by elimination after fan, APS, and flue checks pass.
What you can safely check yourself
If you can safely see the flue terminal (usually on an outside wall or roof), check for obvious blockages — bird nests, debris, or leaves. Do not reach inside the flue terminal yourself.
A frozen condensate pipe can create backpressure in the flue path. Thaw the condensate pipe with warm water and attempt a single reset.
Press and hold the reset button for 3 seconds. If A1 returns on the first startup, call an engineer — do not reset repeatedly. The fan and APS require specialist testing.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Test fan motor speed and current draw — replace fan if output is below specification
- Inspect flue terminal and flue run for blockages or partial disconnection
- Test air pressure switch — replace diaphragm or full switch if faulty
- Inspect APS hose for cracks or disconnection
- Clear blocked condensate trap if restricting flue gas flow
- Check PCB fan speed sensing circuit if all other components test correctly
Fan faults are not directly linked to hard water but are common across South London's older housing stock. Many properties in Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley run Greenstar i and Si boilers that are now 10–15 years old — the fan motor lifespan. Annual servicing includes a fan speed check that catches early motor degradation before it causes an A1 lockout.
How much does a A1 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Worcester Bosch boiler showing A1
If in winter, check and thaw the condensate pipe first.
Check the flue terminal for visible external blockages.
Press and hold the reset button for 3 seconds.
Allow the boiler to run through the fan startup sequence (up to 30 seconds before ignition).
If A1 returns, call a Gas Safe engineer — fan and APS faults require diagnostic equipment.
Fan replacement on a Greenstar boiler is a straightforward repair that typically costs £200–£380 including parts and labour. On a boiler under 10 years old with no other faults, this is unambiguously worth doing. On a boiler over 14 years old with multiple ageing components, Reion will present the comparison honestly at the diagnostic visit.
Common questions about the Worcester Bosch A1 fault
What does an A1 fault mean on a Worcester Bosch boiler?
Can a Worcester Bosch A1 fault be fixed without a new fan?
Why does my Worcester Bosch show A1 in cold weather?
How much does a Worcester Bosch A1 repair cost in London?
Is a Worcester Bosch A1 fault dangerous?
Other Worcester Bosch fault codes
Need a Worcester Bosch engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
