BoilerFaultCodesWhat the manual says
Manufacturer manualSafety Thermostat Activated (Boiler or Flue)
What causes this fault?
This fault occurs when the boiler's internal temperature exceeds a safe limit, triggering a safety switch to prevent damage. It is usually caused by water failing to move around the system quickly enough to carry heat away, often due to a pump failure or a blockage. In some cases, the sensor itself may have failed and is reporting a high temperature even if the boiler is cool.
This fault is more common when owners first turn their heating back on in autumn, as circulation pumps can seize up after being inactive over the summer.
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 25: no call-out fees, no surprise bills.
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