BoilerFaultCodesWhat the manual says
Manufacturer manualShould the appliance not light, check it is not in the overheat lock-out state, by pressing the re-reset button on bottom right hand side of the control box.
What causes this fault?
This fault occurs when the water inside the boiler exceeds a safe temperature limit, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving through the system or a build-up of debris blocking the internal heat exchanger. It can also be caused by a faulty sensor misreading the temperature even if the boiler isn't actually too hot.
This fault is more common in autumn when heating systems are turned back on after a long summer break, often revealing a circulation pump that has seized during the idle months.
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 overheat lock-out state: no call-out fees, no surprise bills.
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All 57 fault codes on the Highflow 300
Seeing a different code? Every code this model can display, in one place.