BoilerFaultCodesTechnical description
Flow / heat exchanger temperature sensor differential too great
What causes this fault?
This fault occurs when the boiler's internal sensors detect that heat isn't being carried away from the burner fast enough, leading to a sudden temperature spike. The most common reasons are a failing circulation pump that isn't moving water properly or a buildup of sludge and debris that has restricted the flow within the heat exchanger. Essentially, the boiler is producing heat, but the water is 'bottlenecked' and cannot circulate into your radiators quickly enough.
This issue often surfaces during the first cold spell of autumn when the heating is turned on after being dormant all summer, which can cause 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.