Glow-worm F17 fault codewhat it means and how to fix it
F17 on a cxi, sxi, SI, CI or CI Plus means the mains supply to the boiler has dropped below 170 volts. The fault is usually the house wiring or the supply, not the boiler.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F17
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 F17 means
F17 is a Hepworth-era code that the later Glow-worm lists dropped. The trade tables for the cxi, sxi, SI, CI and CI Plus give it as "power supply is less than 170v", and the 24CI/18SI manual lists 17 as "power supply <170V". The board measures its own incoming mains and refuses to run the ignition and the fan when the voltage is too low to do so reliably; 170 volts is a long way below the 230 the supply should deliver, so a genuine F17 means something is badly wrong upstream of the boiler rather than inside it.
A loose neutral at the fused spur, a spur shared with a high-draw appliance, a long extension of undersized cable to a boiler in an outbuilding, or a supply fault in the street will all do it, and the code may come and go with the time of day. A board whose voltage measurement has drifted is the other possibility, and the way to separate the two is a meter on the boiler's supply while the code is showing.
The Ultracom, Flexicom and Ultrapower have no F17, nor do the two-digit Ultracom2 and Betacom boards or the current F.xx ranges, which stop on low voltage without a dedicated code. There is nothing in Glow-worm's guides for a householder to do beyond noticing whether other things in the house dim or flicker at the same time, which is worth telling whoever comes — and if the whole house is affected, the electricity supplier rather than a boiler engineer is the call.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Poor connection at the fused spur or plug | A loose neutral or a corroded terminal drops the voltage under load and reads low at the boiler while the rest of the house is fine. The commonest finding and the cheapest. | Common |
| Supply genuinely low | A fault in the street or a heavily loaded local network can pull the whole house below what the boiler will accept, usually at peak times. Lights dim as the code appears. | Occasional |
| Board measuring wrongly | A main board whose supply monitoring has drifted shows F17 on a healthy 230 volts. Confirmed by metering the supply while the code is displayed. | Occasional |
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 spur or terminal remade £80–£150 by an electrician. A main board on a CI-era boiler £200–£300 where still available. A supply fault in the street costs nothing.
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.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
- 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
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.