BoilerFaultCodesTechnical description
Flow/return sensor temperature test
What causes this fault?
This fault occurs when the boiler's sensors detect that the water temperature is rising too quickly or behaving erratically, suggesting the water isn't moving through the system properly. The most common reasons are a failing internal pump that can't push the water around, or a blockage within the heat exchanger. It can also be caused by air trapped in the pipework or sensors that have become faulty and are providing incorrect data to the control board.
This issue is more frequent in autumn when the heating is first turned on after summer, as internal pump parts can seize during long periods 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.