BoilerFaultCodesTechnical description
A blockage in the appliance is preventing the system from flowing as it should.
What causes this fault?
This fault occurs when the boiler's internal sensors detect a massive temperature difference between the water entering and leaving the unit, usually because the water has stopped moving. The most common reason is a failed internal pump or a heavy buildup of sludge and debris blocking the main heat exchanger. Without proper flow, the water inside the unit reaches a boiling point almost instantly, forcing the system to shut down for safety.
This issue is most frequently seen during the first cold snap of autumn when a boiler is switched back on after being idle all summer, often causing a seized pump to finally give up.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.