Your boiler has sensed that water is not circulating properly before it starts the heating process, which is usually caused by an air lock or a blockage.

Pre-Circulation Fault

What causes this fault?

This fault occurs when the boiler's internal sensors detect that water isn't moving through the unit quickly enough during its initial checks. This is usually caused by a large pocket of air trapped inside the pipework or a failing pump that no longer has the power to push water around the system. In some cases, a build-up of sludge or debris can also create a physical blockage that restricts the flow.