BoilerFaultCodesYour boiler has shut down because the water is heating up too quickly or unevenly, suggesting a circulation problem.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical 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.
DIY fix possible
Ensure all radiator valves (TRVs) are turned to the highest settingCheck that the boiler pressure is between 1.0 and 1.5 bar on the displayGently bleed air from your radiators using a radiator key
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F.23 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.