Glow-worm F.49 fault codewhat it means and how to fix it
F.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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.49
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What F.49 means
eBUS is the two-wire link Glow-worm boilers use to talk to compatible room controls and outdoor sensors. F.49 on the Energy, Ultimate, Easicom and Compact is "eBUS fault: voltage too low", and the three causes Glow-worm list are all on that pair of wires: "short circuit on eBUS, eBUS overload or two power supplies with different polarities on the eBUS". The Compact manual turns them into measures — check all eBUS connections, check that the connection works, "check the polarity (±) of the eBUS connections".
The two-digit Ultracom2, Betacom 24a/28a and Betacom2 have F49 as "EBUS voltage fault — fault in EBUS line, short circuit in EBUS connector", with the single instruction "check that the eBus controls are fitted and wired correctly". On the older Ultracom hxi and Flexicom the same fault is F15, "E-bus voltage failure" — a number that on the cxi and CI ranges means the gas valve stepper motor, which is worth knowing before anyone quotes for a gas valve.
The practical value of F.49 for a householder is that it almost always follows a change to the controls: a new thermostat wired the wrong way round, a control that draws more than the bus can supply, or a cable pinched during redecorating. The boiler's own electronics are the last suspect. Glow-worm give no user step, and the terminals are inside the casing, but the history of what was done to the controls is the most useful thing an owner can tell the engineer.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Polarity reversed at a control | Glow-worm's "two power supplies with different polarities". eBUS is polarity-sensitive when more than one powered device is on it; a control wired the wrong way round drags the bus voltage down. | Common |
| Short on the eBUS wiring | A cable nicked by a screw, pinched under a floorboard or stripped too far at a terminal. The bus voltage collapses and the boiler stops listening to its controls. | Common |
| Bus overloaded | Too many powered devices on the pair, or one drawing more than Glow-worm allow. Appears when an extra control or sensor has just been added. | Occasional |
| Board eBUS driver | Reached only after the wiring is proven. A board whose bus supply has failed gives the same code on an otherwise perfect installation. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
45 minutes
A wiring or polarity correction £80–£130. A board, if the bus driver has failed, £250–£300.
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 main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
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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.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
- F15Get it checkedF15 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.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
Common questions about Glow-worm F.49
A new thermostat was fitted yesterday and now the boiler shows F.49. Related?
Almost certainly. Polarity or an overload from the new control are Glow-worm's first causes. Whoever fitted it should check their wiring before a boiler engineer is called.
Will the boiler run without the controls?
Not while F.49 stands — the board treats a collapsed bus as a fault. Once the wiring is corrected the code clears.
My Ultracom shows F15. Is that the same?
On the Ultracom hxi and Flexicom, yes — F15 is "E-bus voltage failure". On the cxi and CI ranges F15 is the gas valve stepper motor, a different fault altogether.
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.