P0.5-0.9 bar High

Potterton P0.5-0.9 bar Fault Code

Also written as P0509BAR — same fault.

The Potterton P0.5-0.9 bar fault code means: your boiler has detected that the water pressure is too low, which prevents the system from running safely and efficiently.. It appears on 2 Potterton models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Water pressure in CH system low

What causes the Potterton P0.5-0.9 bar fault?

This fault occurs when there is not enough water in your heating system to maintain the required pressure for operation. The most common reason is a slow leak from a radiator valve or pipework, though it can also happen if air was recently bled from your radiators without the system being topped back up.

This issue is more frequent in autumn when homeowners turn their heating back on for the first time and discover that pressure has dropped over the summer months.

Typical repair cost £0-120 (including labour)

DIY fix possible

Locate the filling loop (usually a silver flexible hose) underneath or near the boilerSlowly open the one or two valves on the hose until you hear water movingWatch the display until the pressure reaches between 1.0 and 1.5 bar, then close the valves tightly

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Potterton models that show the P0.5-0.9 bar fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Potterton P0.5-0.9 bar 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