BoilerFaultCodesYour boiler has switched itself off because it has reached an unsafe temperature to prevent damage to the internal components.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Overheat thermostat lock-out
What causes this fault?
This fault occurs when the water inside the boiler gets too hot too quickly, triggering a safety sensor to shut the system down. The most common reasons are a pump failure that prevents water from moving, or a blockage caused by sludge and debris trapped inside the main heat exchanger.
This issue often appears when the heating is first turned on in autumn, as pumps can seize up after being idle during the summer months.
DIY fix possible
Check that all radiator valves are fully open to allow water to circulate.Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).Press and hold the reset button for 2-5 seconds to see if the system restarts.
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 0A3 — 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.