Glow-worm F15 fault codewhat it means and how to fix it
F15 is the eBUS voltage on an Ultracom, Flexicom or Ultrapower, and the gas valve stepper motor on a cxi, CI or CI Plus. Know which boiler you have before anyone orders a part.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F15
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.
Which boiler do you have?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
An Ultracom, Flexicom or Ultrapower — reset button on the fascia
eBUS voltage failure — the wiring to your controls.
If a control has just been fitted or changed, tell whoever did it first; otherwise book a Gas Safe registered engineer.
A cxi, sxi, SI, CI, CI Plus or Xtrafast — reset from the On/Off switch
Gas valve stepper motor fault. A gas fault.
Do not reset repeatedly. Book a Gas Safe registered engineer.
What F15 means
F15 is the clearest example of Glow-worm's two older code lists colliding. On the Ultracom cxi, hxi and sxi, the Flexicom cx, hx and sx and the Ultrapower, the trade tables give F15 as "E-bus voltage failure" — the two-wire link to compatible controls has lost its voltage, the fault the current ranges call F.49, and one that almost always traces to a control wired wrongly or a cable damaged.
On the earlier cxi, sxi, SI, CI and CI Plus the same F15 is "gas valve stepper motor fault", the 24CI/18SI manual lists 15 as "stepper motor fault (gas valve)", and the Xtrafast table has "gas mechanism fault" — the motor that positions the gas valve has failed or lost its connection, a gas fault with no user-side answer. A control wiring problem and a gas valve motor have nothing in common except the two digits on the display, and the cost of confusing them is a gas valve quoted for a boiler that needs a thermostat rewired, or the reverse.
The boiler tells you which list applies: the Ultracom, Flexicom and Ultrapower are named on the casing, date from about 2006 onward and have a reset button; the cxi, CI and CI Plus are early-2000s boilers reset from the On/Off switch. Neither meaning appears under F15 on the two-digit Ultracom2 and Betacom boards or on the current F.xx ranges. Glow-worm give the householder nothing to do for either.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| eBUS wiring — Ultracom, Flexicom, Ultrapower | "E-bus voltage failure." A control connected with reversed polarity, a cable nicked or pinched, or too many devices on the bus. Follows a change to the controls more often than not. | Common |
| Gas valve stepper motor — cxi, sxi, SI, CI, CI Plus | "Stepper motor fault (gas valve)." The motor that drives the valve, or its plug and lead. The valve is replaced as an assembly where the motor has failed. | Common |
| Board driving either | On both lists the main board supplies the eBUS or drives the stepper, and a board fault gives the same code. Reached only when the wiring or the valve proves good. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
1 hour
A controls wiring correction £80–£130. A gas valve assembly on a CI-era boiler £250–£320 fitted, where still available.
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 gas valve assembly
£90–£200Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm main PCB
£150–£320Authorised 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.70Get it checkedF.70 means the boiler does not know which model it is — the Device Specific Number is missing, wrong or does not match the coding resistor, usually after a board or display has been changed.Related: electronics and communication
- F.49Get it checkedF.49 is an eBUS fault — the control bus voltage has dropped through a short, an overload or two supplies wired with opposite polarity. It points at the wiring to your controls.Related: electronics and communication
- F.63Get it checkedF.63 is an EEPROM fault — the memory on the main board has failed its check. Glow-worm give one cause, "electronics defective", and one measure, replace the PCB. Nothing else clears it.Related: electronics and communication
- F.64Get it checkedF.64 is the board's analogue-to-digital converter reporting a reading it cannot make sense of — Glow-worm blame a shorted flow or return sensor first and the electronics second.Related: electronics and communication
- F12Get it checkedF12 on a cxi, sxi, CI or Xtramax is the link between the main board and the display board. F.12 on an Energy7 system boiler is a cylinder sensor short. Different generations, both engineer's codes.Related: electronics and communication
- F.65Get it checkedF.65 means the main board itself has got too hot — from something outside it, Glow-worm say first, or because it is failing. The boiler stops to protect its electronics.Related: electronics and communication
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
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 support portal
- Manufacturer technical and service documentation
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.