F.27 Boiler Fault Code: What It Means on Every Brand

F27 is the 'false flame' fault across the Vaillant Group brands: the boiler's flame sensor reports a flame at a moment when there should not be one — typically before ignition or after the gas valve has closed. Because that could theoretically mean gas burning when it should not, the boiler locks out hard. In practice the cause is nearly always electrical — a damp or contaminated electrode leaking signal, or a control board fault — but it is not a code to keep resetting; book an engineer.

Lockout — engineer likely 5 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F.27 means	Severity	DIY?
GWGlow-wormshown as F27	The F27 code indicates that your Glow-Worm boiler has attempted to ignite but its flame sensor cannot confirm a stable burner flame is present. Full guide →	Lockout — engineer likely	No
HHeatlineshown as F27	The F27 fault code appears when the boiler's flame supervision system detects an anomaly with the burner flame — specifically, the boiler has opened the gas valve and attempted to light,… Full guide →	Lockout — engineer likely	No
PProtherm	The F.27 code means your Protherm boiler has detected a flame signal at a point when it shouldn't exist — or the flame supervision circuit has reported an unexpected flame reading during… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.27 fault code on Saunier Duval Condens boilers signals one of two closely related problems, depending on the exact model variant. Full guide →	Lockout — engineer likely	No
VVaillant	The F.27 code appears on Vaillant ecoTEC Pro and Plus boilers when the boiler's control system detects what it considers an incorrect flame signal — either a flame is sensed when there… Full guide →	Lockout — engineer likely	No

F.27 brand by brand

Glow-worm F27

The F27 code indicates that your Glow-Worm boiler has attempted to ignite but its flame sensor cannot confirm a stable burner flame is present. As a safety measure, the boiler immediately locks out and shuts down all operation — both heating and hot water — to prevent any possibility of unburned gas accumulating. The boiler will not restart on its own; the underlying fault must be resolved before a reset will hold. On some Glow-Worm models, F27 appears alongside F16, and both relate to the same family of flame-detection faults — if your display is showing F16 rather than F27, the causes and remedies described here apply equally.

Causes, fixes & costs for Glow-worm F27 → · All Glow-worm codes

Heatline F27

The F27 fault code appears when the boiler's flame supervision system detects an anomaly with the burner flame — specifically, the boiler has opened the gas valve and attempted to light, but the flame detection circuit either reports no stable flame or registers a flame signal at the wrong time. The result is an immediate lockout: both central heating and hot water stop working until the fault is cleared. Because Heatline is part of the Vaillant Group, the F27 code carries the same meaning across Heatline and Vaillant boilers — if you have a Capriz, Advance, or Kalder model, the diagnostic approach is identical to the Vaillant ecoTEC range.

Causes, fixes & costs for Heatline F27 → · All Heatline codes

Protherm F.27

The F.27 code means your Protherm boiler has detected a flame signal at a point when it shouldn't exist — or the flame supervision circuit has reported an unexpected flame reading during the ignition sequence. Protherm shares its error code architecture with Vaillant (both are part of the same group), so F.27 carries identical meaning across both brands. In practical terms, the boiler's control board opened the gas valve and attempted to light the burner, but the flame-sensing circuit reported something inconsistent — either a flame was detected before ignition was attempted, or the signal from the sensing electrode was unreliable or out of sequence. Because a rogue flame signal can indicate a dangerous condition (such as a gas valve that isn't closing fully), the boiler immediately locks out as a safety measure, cutting both central heating and hot water until the fault is cleared and the root cause investigated.

Causes, fixes & costs for Protherm F.27 → · All Protherm codes

Saunier Duval F.27

The F.27 fault code on Saunier Duval Condens boilers signals one of two closely related problems, depending on the exact model variant. On some Condens models it indicates a vortex sensor fault — the sensor that monitors gas-air mixing in the venturi tube is reporting an anomaly. On others, it means the boiler's control board has detected a flame signal at a point in the firing sequence when no flame should be present, commonly called false flame detection or flame simulation. Either way, the boiler locks out as a safety precaution, leaving you without heating or hot water until the underlying cause is diagnosed and resolved. Because both issues relate directly to gas combustion and flame control, F.27 should always be taken seriously — particularly if you notice any smell of gas near the boiler or flue.

Causes, fixes & costs for Saunier Duval F.27 → · All Saunier Duval codes

Vaillant F.27

The F.27 code appears on Vaillant ecoTEC Pro and Plus boilers when the boiler's control system detects what it considers an incorrect flame signal — either a flame is sensed when there should be none, or the flame isn't confirmed when the burner fires. Vaillant refer to this internally as a 'flame simulation' safety shutdown. The result is a full boiler lockout: your heating and hot water stop working until the underlying fault is identified and resolved. The code is most frequently encountered on ecoTEC models built between 2005 and 2012, though it can appear on newer units too.

Causes, fixes & costs for Vaillant F.27 → · All Vaillant codes

Frequently asked questions

Is the F.27 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F.27 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F.27 fault myself?

Not usually. On every brand listed, F.27 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F.27 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.