BoilerFaultCodesYour boiler has too much water in the system, causing the pressure to rise above a safe operating level.
Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now — this fault can be dangerous if ignored.
Technical description: High system water pressure
What causes this fault?
This fault occurs when the internal water pressure exceeds safe limits, often because the filling loop used to top up the system has been left slightly open or is leaking. It can also be caused by a failure in the expansion vessel, which is the part responsible for managing the natural expansion of water as it heats up. Without enough room for this expansion, the pressure spikes and triggers an emergency shutdown.
This issue often surfaces when the heating is first turned on in autumn, as the expansion vessel may have lost its air charge over the summer, causing pressure to spike when the water finally heats up.
DIY fix possible
Check that the filling loop valves are tightly closed.Locate a radiator and use a bleed key to release water into a container until the pressure gauge returns to between 1 and 1.5 bar.Restart the boiler once the pressure is stabilised.
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 40 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.