High Lockout
F08 on Intergas Rapid 32

Your boiler has stopped working because it cannot detect a stable signal from the sensor that monitors exhaust gases, often due to a loose wire or a faulty internal component.

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

Flue sensor open circuit

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the exhaust gases. It is usually caused by a wire vibrating loose over time, moisture affecting the electrical connections, or the sensor itself failing due to constant exposure to high heat. When this signal is lost, the boiler shuts down as a safety precaution to ensure it isn't venting gases at dangerous temperatures.

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

F07View →F06View →F15View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →