Glow-worm F.9 fault codewhat it means and how to fix it
F9 on an Ultracom, Flexicom, Ultrapower or CI-range boiler is Glow-worm's water pressure sensor fault — on the hxi and Flexicom "or a blocked flow filter or pump fault". It is not the low-pressure code.
Take care
What F.9 means
F9 belongs to the F1-to-F77 generation: the Ultracom hxi, cxi and sxi, the Flexicom, the Ultrapower, the Xtramax HE and the CI, CI Plus and SI ranges. Glow-worm's trade tables give two versions of it. For the Ultracom hxi and the Flexicom it is "water pressure sensor fault, blocked flow filter or pump fault"; for the cxi, sxi, CI and SI ranges it is "water pressure sensor fault (please contact GW technical department for further information)".
The 24CI/18SI installation manual lists 09 on its two-digit display as "water pressure sensor fault", and separately lists a flashing pressure reading — "0.5bar", lack of water — for genuinely low pressure, which tells you F9 is the sensor's fault rather than the water's. On the Flexicom the low-pressure code is F22, and its user guide says so. This page used to say that F9 was simply the older name for F22 and could be cleared by repressurising; Glow-worm's current homeowner page does not say that, and no manual we have read says it, so the claim is withdrawn.
What the user guides for these ranges do let a householder do is read the pressure gauge, top up if it is below 1 bar, and press reset once. If F9 stays with the gauge reading normally, the sensor, the filter or the pump is the engineer's to find. The current F.xx ranges have no F.9; their sensor codes are F.73 and F.74.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Water pressure sensor failed or disconnected | Glow-worm's first cause on every range that shows F9. The sensor is a small transducer on the hydraulic block inside the casing; a failed element or a plug knocked off gives the board no pressure it can trust. | Common |
| Blocked flow filter — Ultracom hxi and Flexicom | Named in the hxi and Flexicom tables. Debris caught in the filter starves the sensor of a true reading when the pump runs, and the board reports the sensor rather than the sludge. | Occasional |
| Pump fault — Ultracom hxi and Flexicom | The third clause in the same table entry. A pump that does not run, or runs without moving water, leaves the pressure signal flat where the board expects it to change. Overlaps with F73 on these ranges. | Occasional |
| Pressure genuinely low as well | Not the code's meaning, but a system that has dropped below the sensor's range can show F9 rather than a flashing pressure. The gauge settles it, which is why the first step is to read it. | Occasional |
What to try yourself
Read the pressure gauge on the boiler with the system cold. Glow-worm's Ultracom, Flexicom and Ultrapower user guides want it at or above 1 bar.
If it is below 1 bar, top up through the filling loop as your user guide describes, to between 1 and 1.5 bar, and close and disconnect the loop afterwards.
Press the reset button once. If F9 clears and stays clear, it was the pressure.
If F9 returns with the gauge reading normally, stop there and book an engineer. Tell them the pressure reads correctly, because that points them at the sensor, the filter or the pump rather than the water.
Do not keep resetting or topping up against a reading you cannot trust.
Get a Gas Safe registered engineer to look at your Glow-worm F.9
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 it costs to fix
Typical repair cost
£0–£300
Time on site
1 hour
Nothing if it was low pressure. A pressure sensor fitted £120–£240. A pump on an Ultracom or Flexicom £200–£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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Glow-worm water pressure sensor
£25–£60Authorised 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.22Get it checkedGlow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Related: water pressure
- F.75Get it checkedGlow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.Related: water pressure
- F.73Get it checkedThe water pressure sensor is reading too low to be real — open circuit or shorted — so the boiler has no pressure figure it trusts. On Ultracom boilers F73 is a pump fault instead.Related: water pressure
- F.74Get it checkedThe water pressure sensor is sending a signal too high to be real — a short to the 5 or 24 volt supply, or an internal fault. An engineer's code wherever it shows.Related: water pressure
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: same brand
- F1Get it checkedOn Ultracom, Flexicom and Ultrapower boilers F1 is an ignition lockout after five failed attempts, and Glow-worm's guides let you reset it. On Energy and Ultimate boilers F.01 is the return sensor instead.Related: same brand
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.9
Is F9 the old name for F22?
No. On the ranges that show F9 the low-pressure code is F22 — the Flexicom user guide lists both — and F9 is the pressure sensor itself. An earlier version of this page said otherwise on the strength of a sentence that is not in any Glow-worm document we can find.
The trade table says "contact GW technical department". What does that mean for me?
That Glow-worm did not publish a diagnostic for F9 on the cxi, sxi and CI ranges even for engineers. Expect the engineer to prove the sensor and its wiring, and to ring Glow-worm if that does not resolve it.
Will a magnetic filter help?
On the hxi and Flexicom, where Glow-worm name a blocked flow filter, keeping debris out of the system is the long-term answer. That is a system decision for the engineer, not a Glow-worm instruction for F9.
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.