BoilerFaultCodesPotterton P 0.4 Fault Code
Also written as P04 — same fault.
The Potterton P 0.4 fault code means: your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.. It appears on 1 Potterton model in our database.
Technical description: System pressure dropped below 0.5 bar. The boiler will not light and it will be necessary to top up the pressure.
What causes the Potterton P 0.4 fault?
This fault occurs when the water pressure inside your heating system drops below a safe working level, causing the boiler to shut down to prevent damage. The most common reasons for this are a leak somewhere in your radiators or pipework, or a failure of the internal expansion vessel which helps manage pressure changes.
This issue often appears when you turn your heating back on for the first time in autumn, as the system cools and contracts during the summer months.
DIY fix possible
Locate the silver filling loop underneath or near the boiler.Slowly open the valve or valves until you hear water entering the system.Close the valves once the pressure gauge on the front of the boiler reaches between 1.0 and 1.5 bar.
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Gas Safe Register →Potterton models that show the P 0.4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton P 0.4 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