BoilerFaultCodesTechnical description
Flow/return sensor temperature test.
What causes this fault?
This fault occurs when the boiler's internal sensors detect that water is not moving through the system quickly enough to carry heat away from the burner. The most common reason is a failing circulation pump or a blockage within the heat exchanger caused by a buildup of sludge and debris. Essentially, the boiler is shutting down to prevent itself from overheating because the water has become trapped and is heating up too fast.
This issue often comes to light during the first cold snap of autumn when the heating is turned on after being stagnant all summer, causing internal pump parts to seize.
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.