Glow-worm F.00 fault codewhat it means and how to fix it
The flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.00
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.00 means
The flow sensor is the thermistor clipped to the pipe leaving the heat exchanger, and the board reads it constantly while the burner is alight. F.00 is what an Energy, Ultimate, Easicom or Compact shows when that reading disappears. Glow-worm's Energy7 and Ultimate3 manuals give four causes in one breath: "NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC sensor defective".
The Compact manual calls it "flow temperature sensor interruption" and pairs each cause with a check — the plug, the multiple plug, the harness, and finally the sensor. Glow-worm's homeowner page keeps it to one line, "the flow temperature sensor has been interrupted", and sends the reader to an engineer, because nothing on the list can be reached with the case on.
On the two-digit Ultracom2, Betacom 24a/28a and Betacom2 the same fault is F00, "flow heating temperature sensor fault — sensor open circuit". The Ultracom, Flexicom, Ultrapower and CI generations number the flow thermistor differently: F6 on the trade tables, shown as 06 on a CI-range display, so an owner of one of those boilers should read this page with that in mind. Whatever the number, the fix is the same small part or a reseated plug.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor plug loose or off | Glow-worm's first cause on every current manual. The NTC plug is small and sits where a service engineer's hand passes; one knocked loose reads as an open circuit the moment the board next looks for it. | Common |
| Thermistor failed open | The sensor is a sealed bead that ages with heat. An open-circuit reading against the resistance table in the installation manual confirms it, and it is replaced rather than repaired. | Common |
| Break in the harness or the multiple plug on the board | The two remaining causes in Glow-worm's list. A harness chafed on the casing, or the multi-way plug at the PCB not fully home, gives the same symptom as a dead sensor and is checked first because it costs nothing. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
45 minutes
A flow thermistor fitted £80–£160 including the call-out. A harness repair sits inside the same range; a board is a different job and a different figure.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm flow or return temperature sensor (clip-on NTC)
£12–£35Authorised Glow-worm parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Glow-worm fault codes
- F.10Get it checkedF.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Related: temperature sensors
- F.11Get it checkedOn Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Related: temperature sensors
- F.71Get it checkedThe flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.Related: temperature sensors
- F.13Get it checkedOn a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.Related: temperature sensors
- F.84Get it checkedThe return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.Related: temperature sensors
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.00
Will the boiler run at all with F.00?
No. Without a flow temperature the board has no way to stop the heat exchanger overheating, so it locks out until the sensor is seen again.
My Ultracom shows F6, not F.00. Is that the same thing?
Yes. On the Ultracom, Flexicom, Ultrapower and CI ranges the flow thermistor fault is numbered F6 — 06 on a CI display — and F00 does not appear in those tables. The part and the fix are the same.
Is F.00 the same as F.01?
F.01 is the return sensor, F.00 the flow. Same failure, different pipe. Glow-worm list identical causes for both.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.