BoilerFaultCodesYour boiler is having trouble detecting the steady flame it needs to run safely, but it may just be a temporary sensing error.
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: Ion base value lower than the upper factory limit, try resetting the boiler
What causes this fault?
This fault occurs when the boiler's internal sensor fails to detect a strong enough electrical signal from the flame during operation. It is often caused by a dirty or worn-out sensing electrode, though it can also happen if the burner isn't receiving the correct mix of gas and air. To protect your safety, the boiler stops when it cannot verify that the gas is burning correctly.
DIY fix possible
Locate the reset button (usually a circular arrow or 'Reset' text) on the control panel.Press and hold the reset button for five seconds.Wait for the boiler to restart and monitor if the code clears.
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 12-66 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.