BoilerFaultCodesPotterton P0.5-0.9 Fault Code
Also written as P0509 — same fault.
The Potterton P0.5-0.9 fault code means: your boiler has detected that there is not enough water pressure in the central heating system to operate safely.. It appears on 1 Potterton model in our database.
Technical description: Water pressure in CH system low. Replenish CH system & purge air. Find leak(s) and repair. Compare analogue gauge pressure.
What causes the Potterton P0.5-0.9 fault?
This fault occurs when the water inside your radiators and pipework has dropped below the minimum level required for the boiler to fire up safely. The most common reasons are a slow leak somewhere in the system, air being released through valves, or the internal expansion vessel losing its air charge over time.
This issue is frequently seen in autumn when the heating is turned on for the first time after summer, as dormant leaks or air pockets often become apparent once the system is pressurised and heated.
DIY fix possible
Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.Open the small valves on the filling loop slowly to let water into the system.Watch the pressure gauge and close the valves once the pressure reaches 1.5 bar.
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Gas Safe Register →Potterton models that show the P0.5-0.9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton P0.5-0.9 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