Vaillant F.43 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.43 fault code mean?
The Vaillant F.43 fault code signals a gas group fault, meaning the boiler has detected that the ratio between the gas supply and the flame it produces is outside the acceptable range. In plain terms, the combustion balance is off — the boiler cannot confirm it is burning gas safely and efficiently, so it shuts down as a precaution. This can stem from a configuration mismatch (such as the wrong gas restrictor or an incorrect gas family setting stored on the PCB), a blocked pressure-sensing point inside the venturi, a faulty coding resistor, or moisture that has crept into the electronics — the last of these being particularly common on Vaillant boilers manufactured before 2011. Because this fault touches gas combustion directly, it must be investigated by a Gas Safe registered engineer; homeowners should limit themselves to a single reset attempt while the engineer is arranged.
General guidance only — not a substitute for professional advice. 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
- Wrong gas restrictor or gas family setting Common
If the gas restrictor fitted inside the appliance does not match the gas type being supplied, or the gas family programmed into the PCB does not correspond to the actual supply, the combustion ratio will fall outside Vaillant's permitted limits and F.43 will be triggered. This is one of the most frequently reported root causes, particularly after a service where components have been disturbed.
- Blocked venturi pressure-measuring point Common
The venturi is a precision component that measures gas flow and adjusts combustion accordingly. If the internal pressure-sensing port within the venturi becomes blocked — by debris, corrosion products, or condensate — the boiler loses accurate feedback and faults out. Note that lubricants must never be applied to the O-ring in the venturi, as this can cause exactly this type of blockage.
- Coding resistor or PCB mismatch Sometimes
The coding resistor, located in the wiring harness near the heat exchanger, communicates key configuration data to the PCB. A short circuit, open circuit, or poor connection in that resistor circuit causes the PCB to read an out-of-range gas group value, triggering F.43. The resistor value must match the gas family entered in the boiler's settings.
- Moisture ingress into the electronics Sometimes
Water or condensation reaching the PCB, spark generator, ignition lead, or electrode connections can distort electrical signals and cause the boiler to misread combustion parameters. This is particularly prevalent in older Vaillant models (pre-2011) where sealing around electronics may have deteriorated over time.
- Defective gas valve Rare
If the gas valve is not modulating correctly, it can deliver gas at the wrong rate relative to the air supply, pushing combustion quality outside acceptable limits. A gas valve fault will generally present alongside other combustion-related symptoms and is confirmed by an engineer with a flue gas analyser.
How to fix it
- Check your gas supply is working DIY safe
Before calling an engineer, verify that other gas appliances in your home — such as your hob or gas fire — are working normally. If nothing gas-powered works, contact your gas supplier or the National Gas Emergency Service on 0800 111 999. If your supply is fine, this rules out a wider network issue.
- Reset the boiler once DIY safe
On most Vaillant ecoTEC and turboTEC models, press and hold the reset button (marked with a flame and a diagonal line, or labelled 'Reset') for approximately three seconds until the display changes and the fault code clears. If the boiler fires up, monitor it for at least 30 minutes. Do not reset more than two or three times in total — repeated resets without a lasting fix mean an engineer is needed.
- Note down any pattern to the fault DIY safe
Take a photo of the fault code display and note whether F.43 appears immediately on start-up, after a short run, or only when heating or hot water is called for. This information is genuinely useful to the engineer and can shorten their diagnostic time.
- Gas Safe engineer: verify gas family setting and coding resistor Gas Safe engineer
The engineer should confirm that the gas family programmed into the boiler's PCB matches the actual gas supply type. They will also inspect the coding resistor in the wiring harness at the heat exchanger, testing continuity and resistance with a multimeter, and replace the resistor if values are outside specification.
- Gas Safe engineer: inspect and clean the venturi Gas Safe engineer
The engineer should remove and carefully inspect the venturi for blockages at the internal pressure-sensing port. Any debris should be cleared without the use of lubricants on the O-ring, as contamination here can worsen the fault. If the venturi is damaged or deformed, it will need replacing.
- Gas Safe engineer: check for moisture and inspect ignition components Gas Safe engineer
The engineer should examine the PCB, spark generator, ignition electrode, and ignition lead for signs of moisture, corrosion, or tracking marks. Any affected components should be dried, treated, or replaced as appropriate. On pre-2011 models this step is especially important.
- Gas Safe engineer: combustion analysis and gas valve check Gas Safe engineer
Using a calibrated flue gas analyser, the engineer should measure CO, CO₂, and O₂ levels to confirm combustion quality. If the readings remain outside acceptable limits after addressing the above points, the gas valve modulation should be tested and the valve replaced if it is found to be defective.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
F.43 is a combustion-safety lockout — the boiler has shut down because it cannot verify safe gas burning. If the fault returns after a reset, or if you have any doubt, do not continue using the boiler. Contact a Gas Safe registered engineer. You can find one at gassaferegister.co.uk. If you smell gas at any point, leave the property immediately and call 0800 111 999.
Parts you may need
- Vaillant venturi assembly · from £55
- Coding resistor / gas group resistor · from £25
- Vaillant PCB (printed circuit board) · from £280
- Ignition electrode · from £30
- Ignition lead · from £20
- Gas valve · from £180
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–£400, depending on the underlying cause.
Frequently asked questions
Is F.43 dangerous — can I keep using my Vaillant boiler?
F.43 is a combustion lockout, which means the boiler has already switched itself off as a safety measure. You should not attempt to keep resetting and running the boiler if the fault keeps returning, because the underlying problem relates to how gas is being burned. One careful reset to see whether the fault clears is reasonable, but if it comes back, leave the boiler off and call a Gas Safe registered engineer. If you ever smell gas, do not touch the boiler — leave the property and call the National Gas Emergency Service on 0800 111 999.
Why does F.43 appear more often on older Vaillant boilers?
Vaillant boilers manufactured before around 2011 are more susceptible to F.43 because the sealing and protective coatings around the electronics were less robust than on later models. Over years of heating and cooling cycles, moisture can work its way onto the PCB or ignition components, causing the boiler to misread combustion signals. If your older Vaillant is producing F.43 repeatedly, an engineer may advise that repair costs are approaching the point where replacement is worth considering.
How much does it cost to fix a Vaillant F.43 fault?
Most homeowners pay somewhere between £150 and £400 to resolve F.43. A straightforward fix — such as re-entering the gas family setting, clearing a blocked venturi, or replacing the coding resistor — sits at the lower end of that range. Replacing the PCB is a more involved job and can push costs higher; a Vaillant PCB alone typically costs £200–£350 before labour. Vaillant also offer a fixed-price repair service (around £399 at the time of writing) which covers parts and labour and includes a gas safety check, which can be good value if multiple components need attention.
Can a Vaillant F.43 fault be caused by a recent service or part replacement?
Yes — this is worth flagging to the engineer. F.43 can appear after a service if the wrong gas restrictor was fitted, if the venturi O-ring was lubricated (which Vaillant specifically advise against), or if the gas family setting on the PCB was inadvertently altered. If the fault appeared shortly after maintenance work, the attending engineer should recheck the restrictor size, venturi condition, and PCB configuration as a priority.