High Lockout
F4 on Glow-worm Xtrafast 120

Your boiler has detected an issue with the sensor that monitors hot water temperature, meaning it cannot safely heat water for your taps.

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

Domestic hot water thermistor fault

What causes this fault?

This fault occurs when the internal sensor responsible for measuring the temperature of your tap water fails or sends an incorrect signal to the main control board. The most common reason is that the sensor has physically degraded over time or developed a short circuit, preventing the boiler from knowing how hot the water is. In some cases, the issue may simply be loose or damaged wiring between the sensor and the boiler's computer.

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

F3View →F5View →F6View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

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 →