BoilerFaultCodesTechnical description
Safety thermostat (STB) active: check the flow/content (low-water protection).
What causes this fault?
This fault occurs when the internal temperature sensor detects that the water inside the boiler is getting dangerously hot, triggering an emergency shutdown to prevent damage. The most common reason is a restriction in water flow, often caused by a failing pump or a build-up of sludge and debris blocking the heat exchanger. It can also happen if there is an air lock in the system or if the radiators are turned off, leaving the hot water with nowhere to go.
This fault is more common in autumn when heating systems are turned back on after a long summer break, as internal pump components can seize up from lack of use.
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.