BoilerFaultCodesYour boiler has switched itself off because an external safety device, such as a thermostat or pressure switch, has detected an issue and broken the electrical circuit.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: External interlock circuit is open (e.g., external gas pressure switch or safety thermostat for under floor heating activated); the boiler will shut down.
What causes this fault?
This fault occurs when an external safety component connected to the boiler’s control board detects a potential hazard and breaks the electrical circuit to stop operation. The most common reason is an underfloor heating safety thermostat triggering because the water is too hot, or a gas pressure switch detecting that the incoming fuel supply is unstable.
This issue is more common in winter when high demand on the local gas network can cause pressure fluctuations that trigger external safety switches.
DIY fix possible
Check if your room thermostat or programmer is set too low or has run out of batteriesIf you have underfloor heating, check if the floor has overheated and tripped the safety limitReset the boiler by pressing the reset button for two seconds
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 code9 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Remeha Avanta 18V is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.