High Lockout
F00 on Glow-worm Ultracom 2 HXI

Your boiler has stopped working because it cannot correctly measure the temperature of the water leaving the unit.

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

Flow heating thermistor fault

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the sensor that monitors the temperature of the water sent to your radiators. The most common reason is that the sensor itself has suffered an internal electrical failure, or the wiring connecting it to the brain of the boiler has become loose or damaged. Without this data, the boiler shuts down as a safety precaution to prevent the system from overheating.

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

F01View →F10View →F11View →

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 →