Glow-worm F.89 fault codewhat it means and how to fix it
F.89 means the board cannot see the pump it expects — unplugged, wired wrongly, the wrong pump fitted or a short in its harness. Current ranges only, and it usually follows a pump change.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.89
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.89 means
F.89 is one of the codes added with the current generation of Glow-worm boards, and it is a wiring code rather than a pumping one. The Energy7 and Ultimate3 manuals define it as "Fault: pump" with the causes "pump not connected or it is connected incorrectly, incorrect pump connected, short circuit in the cable harness". The Compact manual gives the same causes as measures and adds the one that matters most in practice: "check whether the pump that is connected is the one that is recommended for the product".
The pumps on these boilers are modulating units that report back to the board over a control lead as well as taking power from it; a pump from another model, or a replacement wired with the power lead but not the signal lead, runs perfectly well and still produces F.89 because the board cannot hear it. It is not in the two-digit Ultracom2 or Betacom tables, nor any older list — those boards drive a simple pump and report circulation problems through F.23, F.24 and F.75 instead.
So F.89 almost always dates from a pump replacement, and the person to call is whoever did it. A pump that is connected and recognised but not moving water shows up as one of the circulation codes, not this one; if the boiler is quiet where the pump should hum and F.89 is on the display, the board and the pump have simply not been introduced. No user guide gives a householder step.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump signal lead not connected after a replacement | Glow-worm's "not connected or connected incorrectly". The pump takes power on one plug and talks to the board on another; a replacement fitted with only the first runs but is invisible to the board. | Common |
| Wrong pump fitted | The Compact manual's "check whether the pump that is connected is the one recommended". A pump from another range or a generic unit does not speak the board's protocol. | Occasional |
| Short in the pump harness | A lead chafed on the pump body or trapped by the casing. Rarer than the two above and found with a meter. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£150
Time on site
45 minutes
A wiring correction £80–£150. If the wrong pump was fitted, replacing it with the correct one is the installer's to put right.
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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
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 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.