High Lockout
F5 on Saunier Duval Isotwin F30E

Your boiler has switched itself off because it has reached an unsafe temperature to protect the internal components from 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

Overheating fault

What causes this fault?

This fault occurs when the water inside the boiler gets too hot, causing a safety sensor to trip and shut the system down. The most common reasons are a pump failure that stops water from circulating or a buildup of sludge and debris that restricts flow through the heat exchanger.

This fault is more common when the heating is first turned on in autumn, as pump components can seize up after being idle throughout 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 →

Related fault codes

F14View →F24View →F25View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →