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Vaillant Boiler F.27 Error Code
Flame detected with the burner off on Vaillant boilers.
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The boiler sees a flame signal at a time when the gas should be off. Vaillant lists a gas solenoid valve that isn't closing tightly, moisture on the electronics, a faulty flame monitor or a defective PCB.
Compatible models
Appears on Vaillant's UK fault code page and in the ecoTEC plus VU 126 to 356/6-5 OVZ fault table; your own manual confirms whether your model uses it.
Also shown as
No other codes are listed for this fault.
Difficulty
Try the basic checks, but the usual fix is a technician's job.
Not sure you are reading the code right? See how to read Vaillant boiler codes.
How to fix Vaillant F.27
Turn the boiler off
Switch it off and close the gas stopcock under the boiler if you can reach it.
Note the code and call
Write down the code before calling Vaillant or an engineer. Don't reset it repeatedly.
When to call a technician
A gas valve that might be letting gas through is a Gas Safe engineer job straight away. If you smell gas, leave the house and call your gas emergency number.
Parts linked to F.27
These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.
Frequently asked questions
Which Vaillant models show the F.27 code?
Appears on Vaillant's UK fault code page and in the ecoTEC plus VU 126 to 356/6-5 OVZ fault table; your own manual confirms whether your model uses it.
Can I fix Vaillant F.27 myself?
Only the outside checks. The steps above are the ones Vaillant's user instructions allow; if they don't clear F.27, the fix is inside the boiler and needs a Gas Safe registered engineer (RGII in Ireland).
Other Vaillant boiler codes
- F.22Water pressure in the heating system is too low
- F.28Boiler failed to light
- F.29Flame went out and the boiler couldn't relight
- F.20Overheat safety cut-out has tripped
- F.23Too big a gap between flow and return temperature
- F.24Temperature rising too fast
- F.25Flue gas temperature too high
- F.75No pressure rise when the pump starts
- F.00Flow or return temperature sensor not reading
- F.10Temperature sensor short circuit
- F.13Hot water cylinder sensor short circuit
- F.32Fan speed out of range
- F.61Gas valve control fault
- F.62Flame took too long to go out
- F.68Unstable flame
- F.70Invalid device code after a repair
- F.83Dry fire: no temperature change when the burner lit
Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.