BoilerFaultCodesPotterton B.(4) Fault Code
Also written as B4 — same fault.
The Potterton B.(4) fault code means: your boiler's pilot light is going out as soon as you let go of the ignition button, preventing the main burner from firing up.. It appears on 3 Potterton models in our database.
Technical description: Main burner fails to light and pilot is extinguished when pressure on the control knob is released - Partially blocked pilot injector.
What causes the Potterton B.(4) fault?
This fault occurs when the pilot flame is too small or weak to properly heat the safety sensor, known as a thermocouple. The most common reason is a buildup of dust or debris inside the pilot injector, which restricts the gas flow and prevents the boiler from detecting that it is safe to stay on. Because the sensor doesn't get hot enough, it automatically cuts the gas supply as a safety precaution the moment you stop pressing the start button.
This issue often comes to light in autumn when the boiler is turned back on after being idle over the summer, as settled dust is more likely to cause a blockage then.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Potterton models that show the B.(4) fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton B.(4) 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