BoilerFaultCodesWhat the manual says
Manufacturer manualSafety Thermostat Activated (Boiler or Flue)
What causes this fault?
This fault occurs when water inside the boiler reaches an unsafe temperature, triggering a safety switch to prevent damage. The most common reasons are a failed pump that isn't circulating water properly, or a buildup of sludge and limescale that blocks the heat exchanger. It can also be caused by a simple sensor failure where the boiler incorrectly thinks it is overheating.
This issue often surfaces when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
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 21: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.