Glow-worm F.74 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.74
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.74 means
Glow-worm call F.74 "electrical problem in the water pressure sensor" and give two causes: the line to the sensor has a short circuit to 5 V or 24 V, or the sensor has an internal fault. The Compact manual states it as the sensor signal "outside correct range (too high)", which is the clearest way to put it — where F.73 is the signal missing or too low, F.74 is the same circuit reading impossibly high.
The boiler treats it as it treats F.73: with no pressure reading it can trust, it will not fire. The Ultracom2, Betacom 24a and Betacom2 installation manuals list F74 as "heating circuit pressure sensor fault, fault in pressure sensor", so the two-digit boilers use the same number for the same thing.
The Ultracom, Flexicom and Ultrapower lists do not carry a 74 at all, so if an F1-to-F77 boiler appears to show it, look again — F7 followed by a flicker, or F4, are the likelier readings. Nothing on this page is a householder task: the sensor and its plug are inside the sealed casing, and every Glow-worm user guide sends codes other than F.22, F.28 and F.83 to a competent person.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor cable shorted to a supply line | Glow-worm's first cause: the signal wire touching the 5 or 24 volt feed inside the harness, usually where the loom passes a sharp edge or has been trapped by the casing during a service. | Common |
| Sensor failed internally | The transducer element drifts high or fails to a rail. Glow-worm name it directly as "internal fault in the water pressure sensor" and it is replaced rather than repaired. | Common |
| Board input damaged | Rarer, but a board whose sensor input has been damaged by a short reads high afterwards even with a new sensor fitted. The engineer proves the sensor first because it is far cheaper. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£260
Time on site
1 hour
Sensor replacement £120–£240. If the board input has been damaged as well, £250 upward.
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 water pressure sensor
£25–£60Authorised 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.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.9Get it checkedF9 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.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.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.74
Is F.74 dangerous?
It is not a gas fault. The risk is only that the boiler cannot protect itself against low water while the sensor is wrong, which is exactly why it refuses to run.
What will the display show for pressure?
Often a figure far higher than the system can hold, or a flashing reading. Do not act on it — do not bleed a radiator to bring an imaginary 3 bar down.
Is this the same as Vaillant F.74?
Glow-worm's own table gives the same meaning and the same two causes, and the two brands share the platform. This page is written from Glow-worm's document.
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.