BoilerFaultCodesTechnical 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.
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.