BoilerFaultCodesYour boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Water pressure switch open (low system pressure)
What causes this fault?
This fault occurs when the pressure of the water circulating through your pipes and radiators drops below a safe level, causing the safety switch to cut power to the boiler. The most common reasons are a slow leak somewhere in the heating system or a loss of air pressure in the expansion vessel, which helps manage water volume as it heats up.
This issue often appears in autumn when the heating is turned back on after summer, as the change in temperature can highlight small leaks or reveal that pressure has slowly seeped away during months of inactivity.
DIY fix possible
Locate the filling loop (usually a silver flexible hose) underneath or near the boilerSlowly open the small valves on the filling loop until you hear water movingClose both valves tightly once the pressure gauge on the boiler reaches between 1 and 1.5 bar
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like E04 — no call-out fees, no surprises.
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Potterton Envoy 30 is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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