This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Vaillant F.68 fault codewhat it means and how to fix it
The flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.68
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.68 means
F.68 is a flame that the boiler can see but cannot trust. The ecoTEC plus and pro tables print "flame signal unstable — flame monitor reports an unstable flame signal" and give a check list that is almost a tour of the combustion circuit: air ratio, gas flow pressure, the condensate route for blockage, the gas injector, the ionisation flow through the cable and electrode, and flue gas recirculation.
The exclusive and the open-vent ecoTEC plus add "air in gas" and "gas flow pressure too low" at the head of the list; the ecoFIT and ecoTEC sustain add an incorrect gas injector. A flame flickers when the mixture feeding it is wrong or when something is disturbing it.
Condensate backing up into the combustion chamber, a flue arrangement that lets exhaust be drawn back into the air intake, or a gas supply that sags under load will all do it, and so will an electrode too corroded to conduct a steady ionisation current. The 2022 CS/1-5 breaks from the rest, giving only "PCB defective — replace the PCB" for F.068. F.68 is not on the 2005 or 2009 ecoTEC tables, nor the 48–64 kW plus, the ecoMAX or the turboMAX plus.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Condensate route blocked | On the ecoTEC plus, pro, exclusive and open-vent lists. Condensate that cannot drain rises into the combustion chamber and disturbs the flame; in winter the blockage is often ice in an external pipe. | Common |
| Gas supply pressure low or air in the gas | Named on the exclusive, open-vent, ecoFIT and sustain tables. A supply that drops under load, or a newly connected gas line with air in it, gives a flame that surges and fades. | Common |
| Flue gas recirculation or wrong air ratio | Exhaust re-entering the air intake through a flue joint or terminal fault dilutes the mixture. Vaillant's "air ratio" and "flue gas recirculation" items; both are flue-side checks. | Occasional |
| Ionisation electrode or its lead | A corroded electrode or a broken lead cannot carry a steady flame current even when the flame is fine. The cheapest repair on the list. | Occasional |
| Board defective | The only cause Vaillant give on the 2022 CS/1-5, and the last one on the ecoTEC sustain. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£400
Time on site
1 hour
Clearing a condensate blockage or replacing an electrode is £90 to £160. Flue or gas supply work depends on what is found; a PCB is £250 to £400 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.
Vaillant ecoTEC ignition and sensing electrode
0020039221£19–£46Authorised Vaillant parts distributor
Price checked 28 September 2026
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Vaillant main PCB (electronics box)
£160–£340Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.28Stop — call an engineerThe boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Related: ignition and flame
- F.29Stop — call an engineerThe boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.Related: ignition and flame
- F.27Stop — call an engineerThe boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.Related: ignition and flame
- F.87Get it checkedThe ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.Related: ignition and flame
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.68
Why is there a carbon monoxide flag on an unstable flame?
Because two of Vaillant's causes, flue gas recirculation and a wrong air ratio, are combustion faults, and an unstable flame is a flame burning outside its design. The boiler has stopped, which is right. Make sure you have a working CO alarm and do not keep resetting.
Could it be the condensate pipe?
Yes, and in freezing weather it is the first thing to look at. A frozen external condensate pipe is on Vaillant's list for this code, and thawing it with warm water is something Vaillant's own page shows householders how to do. If the code returns once the pipe is clear, book an engineer.
Is the electrode a DIY job?
No. It is behind the sealed combustion chamber and Vaillant's instructions restrict that to a competent person. It is a small, inexpensive part for an engineer to change.
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.