BoilerFaultCodesGlow-worm F01 Fault Code
The Glow-worm F01 fault code means: your boiler has stopped working because a sensor that measures the temperature of the water returning to the unit has developed a fault or disconnected.. It appears on 10 Glow-worm models in our database.
Technical description: Return NTC fault
What causes the Glow-worm F01 fault?
This fault occurs when the internal control board can no longer read the temperature signal from the pipe that carries water back into the boiler. The most common reason is that the sensor has physically failed due to age or internal wear, but it can also be caused by a loose connection or damaged wiring. Because the boiler cannot accurately track the water temperature, it shuts down as a safety precaution.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the F01 fault code
Click a model for the full fault code list and model-specific repair notes.
- Ci Plus Repair: £100-£250
- Flexicom 18sx Repair: £90-160
- Flexicom hx Repair: £90-160
- Flexicom sx Repair: £90-160
- Ultracom 2 CXI Repair: £90-160
- Ultracom 2 HXI Repair: £90-160
- Ultracom 24cx 30cx Repair: £90-160
- Ultracom 35 Store Repair: £90-160
- Ultracom cxi Repair: £90-160
- Ultracom hxi Repair: £90-160
Source
Glow-worm F01 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026