BoilerFaultCodesTechnical description
Microbore system or pump speed setting fault
What causes this fault?
This fault occurs when the boiler detects that water isn't moving through the system fast enough to carry heat away from the heat exchanger. The most common reasons are a failing internal pump that has lost its pushing power or a blockage in the narrow pipework that is restricting flow. When the water sits still for too long, it overheats rapidly, forcing the boiler to shut down for safety.
This issue is most frequently noticed when the heating is first turned on in autumn, as pump components can seize up or sludge can settle during the summer months of inactivity.
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.