BoilerFaultCodesWhat the manual says
Manufacturer manualFlame 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Avoid the next call-out bill
A cover plan would handle faults like H4: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Related fault codes
All 57 fault codes on the Highflow 300
Seeing a different code? Every code this model can display, in one place.