BoilerFaultCodesYour boiler has detected that the water pressure is too low or there is trapped air in the system, preventing it from running safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Insufficient water pressure; Air in boiler. Appears if the pressure in the boiler is low.
What causes this fault?
This fault occurs when the internal pressure sensor detects that there isn't enough water in the system to operate safely. The most common reasons are a slow leak in the pipework, air trapped inside the boiler's heat exchanger, or simply needing a top-up after bleeding your radiators.
This code often appears in autumn when homeowners bleed their radiators for the first time in months, causing the system pressure to drop.
DIY fix possible
Locate the filling loop (the flexible silver hose) underneath or near the boiler.Slowly open the valve(s) to let water into the system until the pressure gauge on the boiler reads between 1.0 and 1.5 bar.Close the valves tightly and check if the '04' code has cleared; you may need to press the 'Reset' button.
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A boiler cover plan would cover faults like 04 — no call-out fees, no surprises.
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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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