Vaillant F.93 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.93 fault code mean?
The F.93 fault code on Vaillant boilers indicates that the combustion regulation system has detected the air-to-gas mixture being burned is outside safe or acceptable limits. In plain terms, the boiler is not burning gas efficiently or cleanly enough, so it shuts itself down as a precaution. This fault requires investigation by a Gas Safe registered engineer — the possible causes range from a blocked venturi to a faulty gas valve, and none of these can be safely addressed by opening the boiler casing yourself.
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
- Blocked, wet, or faulty venturi / mass flow sensor Common
The venturi and its associated mass flow sensor measure the volume of air entering the combustion chamber. If the venturi is dirty, damp inside, or has a blocked filter beneath it, the boiler cannot accurately regulate the air-to-gas ratio. Crucially, the internal pressure measuring point inside the venturi must never be lubricated — even a small amount of moisture or oil can cause a persistent F.93.
- Incorrect gas restrictor fitted Common
Vaillant boilers are set up for a specific gas family, and the gas restrictor must match that family. If the wrong restrictor has been installed — for example, after a service or gas conversion — the flow of gas to the burner will be incorrect, producing poor combustion and triggering this fault.
- Low gas supply pressure Common
If the gas pressure arriving at the boiler is too low, the burner cannot achieve the right mixture. This could stem from a supply issue affecting the property, a partially closed service valve, or a problem at the meter. Checking whether other gas appliances are working normally is a useful first step.
- Incorrect or mismatched coding resistor Sometimes
The coding resistor on the PCB tells the boiler which gas family it is operating on. If this resistor does not match the actual gas type in use, or if the gas family check was completed incorrectly during commissioning or a recent service, the combustion control logic will not be calibrated correctly.
- Defective gas valve Sometimes
A gas valve that is worn, sticking, or failing to modulate correctly will deliver inconsistent gas flow to the burner, resulting in poor and variable combustion quality. This is one of Vaillant's own confirmed causes for F.93 and typically requires the valve to be replaced.
- Flue gas recirculation Rare
If the boiler's flue outlet is positioned such that exhaust gases are drawn back towards the air intake — due to a poorly sited flue terminal, wind conditions, or nearby obstructions — the oxygen content of the incoming air is reduced, degrading combustion quality.
How to fix it
- Check your gas supply is working DIY safe
Try other gas appliances in your home — the hob, gas fire, or another boiler if present. If nothing is working, check whether you have credit on a prepayment meter, or contact your gas supplier. If gas is flowing normally elsewhere, the supply is not the issue.
- Check for a frozen condensate pipe (in cold weather only) DIY safe
If temperatures are at or below freezing, the condensate pipe that runs outside your property may be iced up. You can safely pour warm (not boiling) water over the external section of plastic pipe to thaw it, then attempt a reset. This is unlikely to be the direct cause of F.93 but can contribute to boiler lockout.
- Attempt a boiler reset DIY safe
Press and hold the reset button (or follow the reset sequence in your specific Vaillant model's user guide) for approximately three seconds. If the boiler fires up, monitor it over the next hour. If F.93 returns quickly, do not keep resetting — further resets will not resolve an underlying fault and may mask a safety issue.
- Engineer: inspect and service the venturi and mass flow sensor Gas Safe engineer
A Gas Safe engineer should remove and carefully inspect the venturi assembly and the filter beneath it for blockages, moisture, or debris. If cleaning is needed, no lubricant must be applied to the O-ring or the internal pressure measuring point. A damaged or persistently wet sensor will need to be replaced.
- Engineer: verify gas pressure and restrictor sizing Gas Safe engineer
The engineer should measure dynamic gas pressure at the boiler and confirm it falls within Vaillant's specified range. They should also check that the gas restrictor fitted corresponds to the correct gas family for the installation.
- Engineer: check and confirm the coding resistor and gas family setting Gas Safe engineer
The engineer will verify that the coding resistor on the PCB matches the gas type in use and that the gas family check was completed correctly. If there is a mismatch, the correct resistor will be fitted and the gas family re-entered into the boiler's settings.
- Engineer: test or replace the gas valve if required Gas Safe engineer
If all other checks are satisfactory and F.93 persists, the engineer will assess whether the gas valve is modulating correctly. A defective valve will need replacing — this is a gas-carrying component and must only be handled by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the basic homeowner checks above have not resolved the fault, contact a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. All work on gas-carrying components, the PCB, and the combustion system must be carried out by a registered professional under the Gas Safety (Installation and Use) Regulations 1998.
Parts you may need
- Vaillant venturi assembly · from £55
- Vaillant mass flow sensor · from £70
- Venturi filter / mesh · from £12
- Vaillant gas valve · from £120
- Coding resistor · from £20
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.93 myself by resetting the boiler?
A reset is worth trying once or twice — occasionally a one-off combustion anomaly will clear with a reset and the boiler will run normally afterwards. However, if F.93 returns after resetting, you should not keep repeating it. The underlying fault involves the combustion system, gas valve, or related components, none of which a homeowner should work on. Call a Gas Safe registered engineer to diagnose the root cause.
How much does it cost to repair a Vaillant F.93 fault in the UK?
For most households, the repair falls somewhere between £120 and £350 including labour and parts — this covers the most common fixes such as cleaning or replacing the venturi and mass flow sensor, or adjusting gas pressure. A gas valve replacement tends to sit towards the upper end of that range. If the PCB turns out to be at fault, costs can rise to £400–£600, though PCB failure is a less frequent cause of F.93 specifically. Vaillant also offers a fixed-price repair package (currently around £399 inc. VAT) that includes a gas safety check and covers parts and labour regardless of what needs replacing.
Why does my Vaillant boiler keep showing F.93 after being reset?
A recurring F.93 strongly suggests a physical component fault rather than a one-off glitch. The most common culprits are a venturi that is partially blocked or has been contaminated with moisture or lubricant, a gas pressure issue, or a misconfigured coding resistor following a service. Repeated resets without fixing the underlying problem will not help and could mask a deteriorating situation — an engineer should inspect the boiler promptly.
Is the Vaillant F.93 fault dangerous?
The boiler locks itself out when F.93 appears precisely because poor combustion carries risks including incomplete gas combustion and the potential production of carbon monoxide. Provided the boiler has locked out and is not running, the immediate danger is low. However, you should not attempt to override the lockout or open the boiler casing. Ensure your carbon monoxide alarm is working, and arrange for a Gas Safe engineer to attend as soon as reasonably possible.