BoilerFaultCodesPotterton LOCKOUT: b Fault Code
Also written as LOCKOUTB — same fault.
The Potterton LOCKOUT: b fault code means: your boiler has switched itself off because it has gotten too hot, likely due to a blockage or a pump issue stopping the water from circulating.. It appears on 2 Potterton models in our database.
Technical description: OVERHEAT
What causes the Potterton LOCKOUT: b fault?
This fault occurs when the internal temperature of the boiler exceeds a safe limit, causing the safety sensors to shut the system down instantly. The most common reason is that the water isn't moving through the unit quickly enough to carry the heat away, often due to a failed pump or a build-up of sludge blocking the pipes. In some cases, a faulty sensor may also be misreading the temperature and triggering a false alarm.
This issue often surfaces during the first cold spell of autumn when the heating is turned back on and a pump that has been idle all summer fails to start.
DIY fix possible
Check that your radiator valves are fully open to allow water flowEnsure there is enough water pressure in the system (check the gauge)Press the reset button once the boiler has cooled down
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Gas Safe Register →Potterton models that show the LOCKOUT: b fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton LOCKOUT: b fault code information on this page is sourced and cross-checked against the official Potterton service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026