BoilerFaultCodesTechnical description
Safety switch-off – temperature difference too great (NTC1/NTC2)
What causes this fault?
This fault occurs when the boiler's internal sensors detect that water is heating up too fast or that there is a massive temperature gap between the water leaving and returning to the unit. The most common reason is a pump failure or a blockage that prevents water from moving through the system quickly enough to carry the heat away. It can also be caused by air trapped in the heat exchanger or a buildup of limescale restricting the flow.
This issue often surfaces during the first cold spell of autumn when the heating system 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.