BoilerFaultCodesYour boiler has failed to light or cannot detect that a flame has started, meaning you will not have any heating or hot water until it is fixed.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: No flame signal is present.
What causes this fault?
This fault occurs when the boiler tries to ignite but fails to establish or maintain a stable flame. The most common reasons include worn-out ignition spark probes, a faulty gas valve, or a lack of gas reaching the unit. In some cases, the boiler may actually be lit, but a dirty sensing electrode prevents the system from 'seeing' the flame, causing it to shut down for safety.
This fault is more common during extreme cold snaps, as freezing temperatures can block the external condensate discharge pipe, causing the boiler to fail during the ignition process.
DIY fix possible
Check if other gas appliances, like a hob, are working to ensure your gas supply is active.If you have a prepaid gas meter, check that it has enough credit.Press the 'Reset' button on the boiler control panel once to see if the fault 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 F4 — no call-out fees, no surprises.
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Viessmann Vitodens 100 Compact 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.
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