Glow-worm F92 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F92 fault code mean?
The F92 (sometimes displayed as F.92) fault code appears on Glow-Worm boilers that share the Vaillant ecoTEC-derived platform — typically newer or higher-specification models. It signals that the boiler's control board has detected a mismatch between the combustion quality and the gas group it has been configured for. In practical terms, the PCB's coding resistor setting does not align with the gas type being supplied, or the combustion parameters have drifted outside the acceptable range. Because this represents a potential unsafe combustion condition, the boiler locks out automatically as a safety measure. Note that this code does not appear on all Glow-Worm ranges — older Betacom, Flexicom, and Energy series models use a different fault-code set — but if your display is showing F92 or F.92, this page covers everything you need to know.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Incorrect or faulty coding resistor on the PCB Common
Each Glow-Worm boiler on the Vaillant shared platform uses a small coding resistor fitted to the PCB to tell the control board which gas group the appliance is configured for. If this resistor has failed, become dislodged, or was incorrectly specified during installation or a previous repair, the board cannot verify combustion is within safe limits and raises F92.
- Wrong gas family or group entered in boiler settings Common
If the boiler's gas family configuration was never set correctly — or was inadvertently changed during a service — the control board will flag a mismatch even when the hardware is fine. This can also occur after a PCB replacement if the gas group was not re-entered before commissioning.
- Blocked venturi or mass flow sensor Sometimes
A partially blocked filter beneath the venturi, or a contaminated mass flow sensor, can cause the boiler to measure combustion air incorrectly. This makes it appear as though combustion quality is outside the permitted range even when the gas type setting is correct. Importantly, lubricant must never be applied to the venturi O-ring, as this can cause the blockage in the first place.
- Flue gas recirculation Sometimes
If the flue terminal is positioned too close to a wall, an air intake, or another obstruction, exhaust gases can be drawn back into the combustion air supply. This contaminates the air-fuel mixture and pushes combustion parameters out of range, triggering F92. It is more common on installations where the flue has been extended or redirected.
- Low gas supply pressure Rare
If the incoming gas pressure at the boiler is below specification — due to a meter, regulator, or supply issue — the combustion regulation system may detect a fault and raise F92. This is less common but worth ruling out, particularly if other gas appliances in the property also seem to be running poorly.
How to fix it
- Check that your gas supply is working normally DIY safe
Before anything else, confirm that other gas appliances in your home — such as a gas hob or fire — are working as expected. If you have a prepayment gas meter, make sure there is sufficient credit. If there is no gas supply at all, contact your gas supplier rather than your boiler engineer.
- Attempt a boiler reset (no more than 2–3 times) DIY safe
Locate the reset button on your Glow-Worm boiler — usually a flame symbol or a dedicated reset key on the control panel. Press and hold it for a few seconds until the boiler attempts to restart. If the F92 code returns immediately or after a short while, do not keep resetting repeatedly, as this will not address the underlying fault and could mask the issue for a diagnostic engineer.
- Do not open the boiler casing or interfere with any internal components Gas Safe engineer
F92 is a gas combustion and coding fault. Internal components including the PCB, coding resistor, gas valve, venturi, and mass flow sensor must only be handled by a Gas Safe registered engineer. Attempting to access these yourself is unsafe and may invalidate any remaining warranty.
- Have a Gas Safe engineer test and correct the coding resistor Gas Safe engineer
An engineer will check that the correct coding resistor is fitted to the PCB and that its resistance value matches the gas group for your property (natural gas or LPG). If the resistor has failed or is the wrong specification, they will replace it and re-run the gas family check through the boiler's service menu.
- Engineer to verify and re-enter the gas family configuration Gas Safe engineer
Using the boiler's diagnostic menus, the engineer will confirm the gas family is set correctly and complete the configuration procedure in full. An incomplete or interrupted gas family check is a known cause of F92 and is straightforward to resolve once identified.
- Engineer to inspect the venturi, filter, and mass flow sensor Gas Safe engineer
The engineer will check the filter beneath the venturi for debris or contamination and inspect the mass flow sensor for signs of fouling or damage. These components affect how the boiler measures combustion air, and a blocked or defective part here can trigger F92 even when the gas settings are correct.
- Engineer to check the flue for recirculation issues Gas Safe engineer
The engineer will inspect the flue terminal location and condition to ensure exhaust gases cannot re-enter the air supply. If the flue is too close to a wall or opening, or if there is any physical obstruction or damage, this will need to be rectified before the fault can be permanently cleared.
- Engineer to measure gas supply pressure at the boiler Gas Safe engineer
If the above checks do not identify the cause, the engineer will measure the gas pressure at the boiler inlet to confirm it is within the manufacturer's specified range. If pressure is low, this may point to a regulator or supply issue that requires the gas supplier or a further investigation.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If resetting the boiler does not clear F92, or if it returns after clearing, book a visit from a Gas Safe registered engineer as soon as possible. This fault will not resolve itself and continued lockouts in cold weather can leave you without heating or hot water. You can find registered engineers at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- PCB coding resistor · from £15
- Mass flow sensor · from £85
- Venturi assembly · from £65
- Gas valve · from £140
- Printed circuit board (PCB) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F92 fault dangerous?
Because F92 is a combustion safety lockout, the boiler shuts itself down rather than continuing to operate with potentially incorrect combustion. This means the immediate risk is low — the boiler is not running unsafely. However, you should not attempt to override the lockout or repeatedly reset the boiler hoping it clears on its own. The underlying cause needs to be identified and corrected by a Gas Safe registered engineer before the appliance is safe to run normally again.
Can I fix the F92 fault myself?
The only things a homeowner can safely do are confirming the gas supply is on, attempting a reset up to two or three times, and checking nothing obvious has changed (such as a prepayment meter running out of credit). Everything else — including touching the PCB, coding resistor, venturi, gas valve, or flue system — must be carried out by a Gas Safe registered engineer. This is both a legal requirement for gas work and a genuine safety matter.
How much does it typically cost to repair a Glow-Worm F92 fault?
For the most common causes — correcting the gas family setting, replacing the coding resistor, or cleaning the venturi — most homeowners pay somewhere between £150 and £300 including call-out and labour. If the mass flow sensor needs replacing, costs are typically in the £250–£350 range. If the fault turns out to require a full PCB or gas valve replacement, you could be looking at £350–£500 or more, but these outcomes are less common for F92 specifically. It is worth getting a fixed-price quote before authorising any major parts.
Why does my Glow-Worm boiler show F92 but it is not listed in my manual?
The F92 code originates from the Vaillant group platform that underpins certain higher-specification and newer Glow-Worm models. Older Glow-Worm ranges such as the Betacom and standard Flexicom use a different diagnostic system with different code numbering, so F92 may not appear in the manual supplied with your boiler. If your display is showing F92 or F.92, this page applies to your appliance regardless of whether your manual lists it.