BoilerFaultCodesYour boiler is sensing a flame even though it hasn't been asked to turn on, which is often caused by moisture or a sensor fault.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flame detected with no demand. Gas valve. Flame sense electrodes/cables. Moisture on control board. Blocked condensate trap. Otherwise replace control board.
What causes this fault?
This fault occurs when the boiler's internal sensors detect a flame while the unit is supposed to be off, which is a significant safety conflict. The most common reason is moisture or dampness affecting the control board or electrodes, but it can also be caused by a gas valve that isn't closing fully or a blocked drainage system causing water to back up into the combustion chamber.
This issue is more frequent during periods of heavy rain or extreme cold, as external moisture or a blocked condensate pipe can cause water to build up inside the appliance.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like H4 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Worcester Bosch Highflow 400 BF is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.