BoilerFaultCodesPotterton B(5) Fault Code
Also written as B5 — same fault.
The Potterton B(5) fault code means: your boiler's pilot light has gone out and will not stay lit, meaning the system cannot fire up to provide heating or hot water.. It appears on 3 Potterton models in our database.
Technical description: Failure of thermocouple, power unit or latching mechanism
What causes the Potterton B(5) fault?
This fault occurs when the boiler's safety system can no longer detect the small permanent flame used to light the main burner. The most common reason is that the heat-sensing probe has worn out over time or the internal magnetic switch within the gas valve has failed, causing the gas supply to shut off for safety. Without this pilot light being confirmed as active, the boiler is prevented from firing up entirely.
This issue is frequently noticed during the first cold spell of autumn when the boiler is worked harder after being mostly idle during the summer months.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Potterton models that show the B(5) fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton B(5) 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