Glow-worm F.53 fault codewhat it means and how to fix it
F.53 on an Energy 35 Store is the mass flow sensor — low gas pressure, a wet or blocked Venturi filter, or the sensor itself. F.52 and F.56 are the same sensor's connection and regulation.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.53
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.53 means
The Energy 35 Store meters the gas it burns with a mass flow sensor in the Venturi, and three codes belong to that sensor. The installation manual gives F.53 as "mass flow sensor fault" with the causes "gas flow pressure too low, filter under Venturi filter cap wet or blocked, flow sensor defective, internal pressure measuring point in Venturi blocked", and adds a parenthesis for engineers — "do not use lubricant on Venturi O-ring" — because grease on that seal is how the measuring point gets blocked.
F.52 is "mass flow sensor connection fault — flow sensor not connected/disconnected, plug not connected or incorrectly connected", and F.56 is "fault: mass flow sensor regulation — gas valve defective, cable harness to gas valve defective". No other Glow-worm range has a mass flow sensor and none lists these codes: the Energy7, Ultimate, Easicom, Betacom and Compact boards set the gas valve from the fan speed and have no equivalent. The wet-filter cause is the one that matters most in practice.
The filter sits under a cap on the Venturi and traps debris from the gas supply; moisture reaching it — from a damp installation or a flooded meter box — swells it and chokes the flow reading, and the boiler reports the sensor rather than the water. Glow-worm's operating instructions give the householder nothing for F.53 beyond the general reset; the Venturi is behind the casing and the filter, the sensor and the O-ring are engineer's parts, so this is a page for knowing what to expect rather than what to do.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Filter under the Venturi cap wet or blocked | Glow-worm's second cause. Debris from the gas supply or moisture swelling the filter medium restricts flow and the sensor reads low. Cleaned or replaced with the cap; the engineer checks why it got wet. | Common |
| Gas flow pressure too low | The first cause, and the overlap with F.54. A closed or partly closed isolator cock, a supply problem or a governor fault gives the sensor less to measure than it expects. | Common |
| Sensor, plug or measuring point — F.52 and F.53 | The sensor itself failed, its plug not connected (F.52), or the internal pressure measuring point in the Venturi blocked — Glow-worm single out lubricant on the O-ring as the way that happens. | Occasional |
| Gas valve or its harness — F.56 | Glow-worm's causes for the regulation fault. The board cannot bring the measured flow to the value it wants because the valve or its lead is defective. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£100–£320
Time on site
1h 30m
A Venturi filter cleaned or replaced £100–£180. A mass flow sensor or a gas valve £250–£320 fitted.
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 gas valve assembly
£90–£200Authorised 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.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: ignition and gas supply
- 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: ignition and gas supply
- F.29Get it checkedThe burner lit and then the flame went out while running. Glow-worm blame the gas supply, flue gas recirculating, earthing, the spark transformer and, on newer manuals, a blocked condensate pipe.Related: ignition and gas supply
- F.61Get it checkedThe board cannot drive the gas valve properly — a short in its wiring, coils shorted to earth, or the electronics. The valve stays shut until an engineer has traced it.Related: ignition and gas supply
- F.27Stop — call an engineerThe boiler is sensing a flame when there should not be one. Glow-worm list moisture on the electronics, a failed flame monitor or a leaking gas valve. Not a reset job.Related: ignition and gas supply
- F.68Get it checkedThe flame is burning but its signal is unstable. Glow-worm list air in the gas, low gas pressure, a wrong combustion setting or injector, a poor electrode connection or a blocked condensate pipe.Related: ignition and gas supply
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.53
Can I check the gas pressure myself?
Only indirectly — that the isolator cock is open and other gas appliances work, which is the F.54 check. Measuring inlet pressure and opening the Venturi cap are engineer's work.
Why does the manual warn about lubricant?
Because grease on the Venturi O-ring migrates into the pressure measuring point and blocks it, which produces exactly this code. It is a note for the engineer servicing the boiler, and a reason to use one who knows the 35 Store.
Is this the same sensor as F.52 and F.56?
Yes. F.52 is the sensor unplugged, F.53 the sensor or its reading at fault, F.56 the board unable to regulate what it measures. One visit covers all three.
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.