Glow-worm F.70 fault codewhat it means and how to fix it
F.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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.70
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.70 means
Every current Glow-worm board is shared across several models and told which one it is in by a Device Specific Number. F.70 is "invalid product code (DSN)", and the Energy7 manual's cause describes the moment it appears: "display and PCB replaced at same time and Device Specific Number not reset, wrong or missing output coding resistor". The Ultimate3 manual says the same in different words — the screen and PCB replaced together "without reconfiguring the product's Device Specific Number" — and the Compact manual's only measure is "set the correct Device Specific Number".
Glow-worm's homeowner page carries F.70 as "the Device Specific Number (DSN) is incorrect, missing or does not match the coding resistor", adds "interruption in the cable harness" as a cause, and hands it to an engineer. The two-digit Ultracom2, Betacom 24a/28a and Betacom2 have F70 as "user interface incompatible with the main board — incorrect product code", with the instruction to check that the DSN on the screen matches the product code on the nameplate. The older Ultracom hxi and Flexicom list F70 as "software incompatible", and the cxi, CI and CI Plus use F20 for "software incompatibility" — a number that on every later board means overheat, so an older boiler showing F20 should not be read against the F.20 page without checking the range.
F.70 is nearly always the aftermath of a repair: a new board fitted without its number, or a display and board swapped together so that neither holds the configuration. The Energy 35 Store adds a companion code, F.42, "coding resistance fault (possibly in connection with F.70)" — a short or break in the output coding resistor in the harness at the heat exchanger, or the gas type resistor on the PCB — which no other Glow-worm range lists. The nameplate inside the drop-down flap carries the number the engineer needs, and the fix is configuration rather than another part.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| New board or display not configured | Glow-worm's stated cause. The DSN lives in the board and is copied to the display; replace both at once and there is nowhere to copy it from. The engineer enters it from the nameplate. | Common |
| Coding resistor wrong or missing | "Wrong or missing output coding resistor." A small plug-in resistor on some boards that identifies the output; a replacement board supplied without it, or with the wrong one, shows F.70. | Occasional |
| Harness interruption | The homeowner page's second cause. A break in the wiring between display and board stops the number being read, and looks to the board like no number at all. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£150
Time on site
30 minutes
A configuration visit £80–£150. If the fault follows a recent repair, the installer who did it should correct it within that job.
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
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Glow-worm fault codes
- 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
- 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.70
The boiler was repaired last week and now shows F.70. Was the repair done wrongly?
Not necessarily wrongly, but incompletely — the new part was not given the Device Specific Number. Call whoever did the work; it is a configuration step, not another part.
Where is the Device Specific Number?
On the data plate, which on current ranges is on the underside of the boiler or behind the drop-down flap. It is a number for the engineer to enter, not something the user menu changes.
My Ultracom cxi shows F20. Is that overheat?
Not on the cxi, CI or CI Plus — there F20 is "software incompatibility", the equivalent of this code. F20 means overheat on every later board, so check the range before reading the F.20 page.
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.