Your boiler has overheated to a critical level and a safety component has permanently cut the power to prevent serious damage.

THERMAL FUSE LOCKOUT

What causes this fault?

This fault occurs when the internal temperature inside the main heat exchanger has reached an extreme, dangerous limit, causing a one-time safety fuse to blow. The root cause is typically a severe lack of water flow or a significant build-up of debris and limescale that has caused the component to dry-fire and get too hot. Because the fuse is designed to break permanently to protect your home, the boiler cannot be restarted simply by pressing a button.