Glow-worm F.13 fault codewhat it means and how to fix it
On a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.13
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 and display?
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.
F.13 with a stop on a system or regular boiler with a hot water cylinder
Cylinder temperature sensor shorted, or its cable.
Book a Gas Safe registered engineer. Hot water may still be available on a timer while heating waits.
F.13 with a stop on a combi
A combi has no cylinder sensor. The board is configured wrongly or a harness is shorted.
Book a Gas Safe registered engineer to check the Device Specific Number and the harness.
F13 without a stop on an Ultracom, Flexicom, Xtramax or 13 on a CI display
Glow-worm's table names the main PCB, not a sensor.
Book a Gas Safe registered engineer and say which range, so they expect an electronics fault.
What F.13 means
F.13 only has a sensor to point at on a boiler that heats a cylinder. On an Energy, Ultimate or Compact system or regular model wired to a cylinder thermistor, Glow-worm define it as "Short circuit: Temperature sensor for the domestic hot water cylinder", causes "NTC sensor defective, short circuit in the cable harness, cable/housing".
The Energy7 manual adds that F.12 and F.91 flag the same cylinder sensor short on that range, and the Compact manual finishes its F.13 row with "set the correct Device Specific Number" — the board's configuration code — because a combi told it has a cylinder will report a sensor it was never given. On the Ultracom cxi, hxi and sxi, the Flexicom and the Xtramax HE, F13 is "PCB memory or sensing fault"; on the earlier cxi, sxi, SI, CI and CI Plus it is "main PCB connection fault", and the 24CI/18SI manual lists 13 simply as "main board fault".
So the two-digit F13 on the older ranges is an electronics code, not a sensor code, and the cost of the two readings is very different: a cylinder thermistor is one of the cheapest parts on the installation, a main board one of the dearest. The two-digit Ultracom2 and Betacom tables carry no 13 at all.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Cylinder thermistor shorted — system and regular boilers | The sensor sits in a pocket on the hot water cylinder, often the far end of the longest cable on the installation. A failed bead or a cable trapped behind the airing cupboard door both read as a short at the boiler. | Common |
| Wrong Device Specific Number | The Compact manual's last measure for F.13. A board configured for a system boiler on a combi, or after a PCB swap without the number being set, expects a cylinder sensor that does not exist. | Occasional |
| Main board fault — Ultracom, Flexicom, Xtramax and CI | On those ranges F13 means the main PCB, its memory or its connections. Glow-worm's trade tables give no cause beyond the board; an engineer checks its plugs and supply before replacing it. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£350
Time on site
1 hour
Cylinder thermistor fitted £80–£160. A configuration correction is a call-out fee. A main board on an Ultracom or Flexicom £250–£350.
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 flow or return temperature sensor (clip-on NTC)
£12–£35Authorised 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.00Get it checkedThe flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.Related: temperature sensors
- F.10Get it checkedF.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Related: temperature sensors
- F.11Get it checkedOn Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Related: temperature sensors
- F.71Get it checkedThe flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.Related: temperature sensors
- F.84Get it checkedThe return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.Related: temperature sensors
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
- 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.