BoilerFaultCodesTechnical description
External interlock activated
What causes this fault?
This fault occurs when an external safety device or control unit intentionally breaks the electrical circuit to stop the boiler from firing. The most common reason is a faulty zone valve that has failed to open or an external frost thermostat that has triggered to protect the pipework.
This code is more frequent during extreme cold spells if an external frost thermostat is installed or if a zone valve motor seizes after being inactive through the summer.
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.
Find a Gas Safe engineer
Search the official register →