High Lockout
4.5 on Ideal Vogue Combi

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

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

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.

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 →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →