BoilerFaultCodesYour boiler has detected an error while trying to heat your water, preventing the burner from lighting or staying on correctly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: DHW mode – burner on error
What causes this fault?
This fault occurs when the boiler attempts to ignite to provide hot water but fails to detect a consistent flame or correct operation. The most common reason is a communication error between the sensors and the burner, often triggered by a faulty diverter valve sticking or a sensor incorrectly reporting water flow. It essentially means the boiler’s safety controls have stepped in to stop the burner because it cannot guarantee correct operation in hot water mode.
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 S.20 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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