Glow-worm F.73 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.73
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 display are you looking at?
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.73 with a stop, or F73 on an Ultracom2 or Betacom2
Pressure sensor or its wiring. The pressure shown cannot be trusted until it is fixed.
Book a Gas Safe registered engineer. Do not keep topping up on the strength of the display.
F73 on an Ultracom, Flexicom or Ultrapower
Glow-worm's wording is "unable to see pump activate" — a circulation fault rather than a sensor fault.
Check the pressure reads at least 1.0 bar, reset once, and call an engineer if it returns.
What F.73 means
On the current ranges Glow-worm define F.73 as "water pressure sensor not connected or has short-circuited", and the Compact manual sharpens it to a sensor signal "in the wrong range (too low)". The boiler is not saying the pressure is low; it is saying it cannot believe the sensor, because the voltage coming back is one a working sensor never produces.
The causes Glow-worm list are all electrical: an interruption or short in the sensor itself, a short to earth in its supply line, or the sensor having failed internally. That is why topping up does nothing for this code, and why the display may show a pressure that is plainly wrong or 0.0. The Ultracom2 and Betacom2 manuals carry the same code as "heating circuit pressure sensor fault, the sensor is shorted or disconnected", so the two-digit boilers agree.
The Ultracom, Flexicom and Ultrapower do not: their F73 is "unable to see pump activate", the board having run the pump and seen no change in the pressure signal, so on those boilers the code is closer to what the current ranges call F.75. Either way this is an engineer's code — the sensor sits inside the casing on the hydraulic block, and the user guides for every range hand 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 unplugged, chafed or shorted to earth | Glow-worm's wording is "interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line". A plug disturbed during a service, or a cable rubbed through on the casing, is the cheap version of this fault. | Common |
| Pressure sensor failed internally | The sensor is a small transducer screwed into the hydraulic block, and its output drifting out of range is the other half of Glow-worm's cause list. Replaced as a unit, with the system drained down to the boiler. | Common |
| Pump not proving pressure — Ultracom and Flexicom only | On the F1-to-F77 boilers F73 means the pump ran and the pressure signal did not move. A seized pump, an airlocked one or a sensor that cannot see it are the possibilities, and the older cause list for F9 overlaps. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£240
Time on site
1 hour
Water pressure sensor replaced £120–£240 including the drain-down and refill. A wiring repair alone is at the bottom of that range.
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
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.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.73
Can I top up to clear F.73?
Not on a current range. The code says the sensor cannot be read, not that pressure is low, so adding water changes nothing and risks overfilling a system you cannot see the pressure of.
How is F.73 different from F.74?
Same sensor, opposite direction. Glow-worm define F.73 as the signal too low or absent and F.74 as an electrical problem with the line shorted to 5 or 24 volts — the signal too high. Both are the sensor circuit.
Will the boiler run at all?
No. Without a pressure figure it will not fire, because the low-pressure protection depends on that sensor.
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.