High Lockout
F01 on Glow-worm Ultracom cxi

Your boiler has stopped working because a sensor that monitors the temperature of the water returning from your radiators has developed a fault.

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

Return heating thermistor fault

What causes this fault?

This fault occurs when the boiler's control board receives an incorrect or missing electrical signal from the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor has physically failed due to age, though it can also be caused by loose or damaged internal wiring.

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

F00View →F10View →F11View →

Related guides you might find useful

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 →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →