Vaillant F27 Fault Code: False Flame Signal, Causes & What to Do

Fix & Repair Engineers diagnoses and repairs Vaillant F27 faults across London and the Home Counties. Our engineers investigate the gas valve, PCB, flame-monitoring electronics and monitoring electrode properly before recommending a repair.

The table below summarises the most-asked questions about F27 before the full detail further down the page.

The simplest accurate explanation is that F27 means the boiler believes a flame is present even though the burner should be switched off. Vaillant's current homeowner wording is particularly useful because it avoids vague phrases such as "incorrect flame" or "unstable flame." The manufacturer says "A flame signal has been detected while the burner is off." Professional Vaillant manuals use another important term: "Flame simulation." For example, relevant ecoTEC documentation describes F27 as "Safety switch-off: Flame simulation," with listed causes including moisture on the electronics, defective electronics/flame monitor, and leaking gas solenoid valve. F27 is best understood as a false or unexpected flame-detection fault. The boiler expects no flame, but its safety monitoring system is telling it otherwise. Rather than continuing to operate under contradictory information, the boiler shuts down.

What Does "Flame Simulation" Mean?

"Flame simulation" sounds more complicated than it is. It does not mean the boiler is deliberately creating a simulated flame. It means the electronic safety system is receiving a signal that resembles a genuine flame signal when no normal flame should be present. That can happen for two fundamentally different reasons. First, there really is an unexpected flame condition: a gas valve that does not seal correctly may allow gas to continue reaching the burner when it should have stopped, which is why Vaillant's current homeowner guidance specifically lists "gas solenoid valve not leaktight." Second, the boiler is incorrectly detecting a flame: a problem involving the PCB, flame-monitoring electronics, moisture affecting electronics, or the monitoring electrode on applicable models can create an incorrect flame signal. F27 tells us what the boiler is detecting; it does not by itself tell us whether the signal reflects a genuine flame or a false electronic reading. That is what professional diagnosis needs to establish.

Why Does a Boiler Monitor the Flame?

A modern gas boiler must know whether combustion is actually occurring. During normal operation, its controls manage a sequence involving demand for heat, combustion-air and fan operation, gas-valve operation, ignition, confirmation that a flame has established, continued monitoring while the burner operates, gas shut-off when heat demand ends, and confirmation that the flame has extinguished. The flame-monitoring system is therefore an important safety function. The boiler needs to distinguish between flame present when expected and flame present when it should not exist. F27 concerns that second condition.

Vaillant's current public cause list is remarkably concise: gas solenoid valve not leaktight, and PCB defective. Professional manuals then add model-specific detail around moisture, the flame-monitoring electronics and electrode.

Does F27 Mean the Flame Sensor Is Broken?

Not necessarily. This is one area where much of the current SERP becomes too simplistic. A monitoring electrode or flame-detection component can be relevant on particular Vaillant models, but Vaillant's current homeowner F27 list identifies the gas solenoid valve and defective PCB as the two headline causes. F27 does not automatically diagnose a failed flame sensor. The engineer needs to determine whether a genuine residual flame condition exists, the gas valve is not sealing, the monitoring electrode is reporting incorrectly, moisture is interfering with electronics, or the PCB/flame-monitoring circuit is defective.

Does F27 Mean the Gas Valve Has Failed?

Again, not automatically. A gas solenoid valve that is not leaktight is a manufacturer-listed F27 cause, but Vaillant also explicitly lists the PCB, and relevant professional manuals additionally include flame-monitoring electronics, moisture and the monitoring electrode. The correct sequence is fault code, then diagnostic testing, then confirmed cause, then repair - not F27 straight to replacing the gas valve. This distinction matters because gas-valve replacement can be a significant repair and should not be recommended without evidence.

Does F27 Mean Gas Is Leaking Into My Home?

No. This is a particularly important safety distinction. Vaillant says a gas solenoid valve that is not leaktight can cause F27, but that refers to the internal gas-control function of the boiler. It does not automatically mean that gas is escaping into the room or property. The boiler has detected an abnormal flame-related condition and has locked out. However, because a gas-valve problem is one possible cause, F27 should be professionally investigated.

F27 is a safety-related combustion and flame-monitoring fault, so it should be taken seriously. However, the fault code itself does not prove that gas is leaking into the property, carbon monoxide is present, the burner is still visibly alight, the boiler is immediately dangerous, or the gas valve definitely needs replacing. It means the boiler has detected flame information that is incompatible with the burner being switched off. The safety response is to lock out. Do not bypass the lockout, and arrange professional investigation if the fault persists or returns.

Vaillant's general guidance states that resetting can sometimes clear a boiler fault or temporarily get the appliance working again, though the cause of the lockout should ideally be addressed before resetting except for minor homeowner-resolvable faults, and F27 is not one of those straightforward homeowner repairs. If there is no smell of gas, no carbon monoxide alarm is sounding, and no other obvious emergency condition, you can follow the reset instructions in the user manual for your exact Vaillant boiler. If F27 returns, stop repeatedly resetting it and arrange professional diagnosis. A reset cannot repair a leaking/non-leaktight gas valve, defective flame-monitoring electronics, PCB failure, moisture damage or a defective monitoring electrode.

Why Does F27 Keep Coming Back After Resetting?

Recurring F27 means the underlying condition has not been permanently resolved. Depending on the boiler model, potential explanations include intermittent gas-valve leakage, intermittent PCB/flame-monitoring failure, moisture affecting electronics, an unreliable monitoring-electrode signal, or another model-specific flame-detection problem. If the fault is intermittent, tell the engineer how frequently it occurs, whether it happens after heating or hot-water demand, whether the boiler had just switched the burner off, whether another fault appears before or afterwards, whether there has been a recent leak or water ingress, and whether F62, F67 or another flame-related code has also appeared.

F27 is frequently confused with several related flame and combustion faults. Each describes a distinct diagnostic event in Vaillant's fault-code system.

F27 vs F28: F27 means flame detected when there should be none. F28 means no flame detected when there should be one. Generic advice about checking gas meter credit, whether other gas appliances work, frozen condensate pipes or ignition failure belongs much more naturally to F28 than F27. See our Vaillant F28 fault code guide for the ignition-failure fault.

F27 vs F29: F27 means the burner should be off but a flame signal is detected. F29 means the burner was operating normally, the flame disappeared, and the boiler failed to reignite. See our Vaillant F29 fault code guide for the flame-loss fault.

F27 vs F62: both codes can involve a gas valve that does not stop combustion as expected, but they describe different detected conditions. F27 means flame exists or appears to exist when it should not. F62 means the flame took too long to disappear during the shutdown sequence. The two codes can therefore point towards overlapping components without representing the same fault. See our Vaillant F62 fault code guide for the delayed flame switch-off fault.

F27 vs F67: Vaillant professional documentation defines F67 as "Fault: Electronics/flame," with an implausible flame signal and defective electronics among the relevant diagnostic areas. F27 is specifically about flame being detected when the burner should be off, while F67 points more directly towards the electronics/flame-signal processing side. F27 can involve that electronics path too, but it also carries the specific possibility that the gas valve is genuinely not sealing correctly.

F27 is sometimes grouped with ignition leads, spark electrodes, failed ignition, low gas pressure and blocked condensate pipes. Those are much more strongly associated with F28/F29 and other ignition or combustion faults. F27 is better described as a flame-detection, flame-simulation or false/unexpected flame-signal fault, rather than simply an "ignition fault."

Can Ignition Leads or a Spark Electrode Cause F27?

Vaillant's current homeowner F27 guidance does not list ignition leads as a direct cause. Relevant professional manuals focus on the gas solenoid valve, PCB/flame-monitoring electronics, moisture, and the monitoring electrode on applicable models. Ignition cables and leads appear much more explicitly within Vaillant's F28 and other ignition-fault diagnostic tables. Generic competitor guides frequently list a faulty spark electrode as a headline F27 cause, but Vaillant's current public F27 list does not. However, professional documentation for some Vaillant boilers does tell the engineer to investigate the monitoring electrode where it is reporting an incorrect flame. Ignition/spark electrode and flame-monitoring electrode are related parts of the combustion-control system, but they should not simply be treated as interchangeable terms in every boiler design.

Can Low Pressure or a Frozen Condensate Pipe Cause F27?

Low heating-system pressure is not one of Vaillant's direct F27 causes. Vaillant uses separate faults such as F22 for low water pressure and F75 for pressure-response problems, so checking or topping up system pressure is not particularly useful F27-specific troubleshooting unless another pressure-related fault is also displayed. Similarly, Vaillant does not list a frozen or blocked condensate pipe as a direct F27 cause - frozen condensate pipework is much more closely associated with F28 and F29. An F27 guide should not send homeowners outside to thaw the condensate pipe simply because it is common Vaillant troubleshooting advice elsewhere.

Very little homeowner troubleshooting is useful for F27, because Vaillant's recognised causes are internal safety-related components. Vaillant specifically says F27 is a case for an expert and warns that attempting to solve it yourself can be dangerous.

The precise diagnostic procedure depends on the boiler generation. A competent engineer should identify the exact model and follow the corresponding Vaillant technical instructions.

Does F27 Mean the PCB or Gas Valve Needs Replacing?

No to both. The PCB is a recognised cause, but it is not the only one - Vaillant's F27 diagnostic information also includes gas solenoid valve leakage, moisture on electronics, flame-monitoring electronics and the monitoring electrode on relevant models. Replacing an expensive PCB purely because F27 appears would be poor diagnostic practice; the component should be tested and confirmed faulty. The same applies to the gas valve: it is one of the most important F27 possibilities, but the PCB and flame-monitoring system can create the same fault condition, so it should only be replaced after professional testing establishes that it is responsible.

How Much Does It Cost to Repair Vaillant F27?

There is no single accurate F27 repair price because F27 is not a confirmed-part code. The eventual repair could involve resolving moisture ingress, repairing an electrical connection, replacing or repairing flame-monitoring components, replacing a monitoring electrode on an applicable model, replacing the gas valve, or replacing the PCB. These have very different parts and labour costs, so published price ranges should be treated cautiously. We diagnose the actual F27 cause first, then quote for the repair required.

Is an F27 Repair Worth Doing on an Older Vaillant?

Potentially, yes. F27 does not automatically make a boiler uneconomical to repair. If the problem is a repairable electronic issue, a monitoring component, moisture-related damage that is limited in extent, or a replaceable gas valve, and the rest of the boiler remains in good condition, repair may make commercial sense. Replacement becomes more relevant where the boiler has suffered repeated major failures, the PCB repair is substantial, multiple major components are deteriorating, significant internal water damage exists, parts availability is poor, or the overall repair cost is difficult to justify. The decision should follow diagnosis rather than be driven by an arbitrary boiler-age threshold.

Can Servicing Prevent Vaillant F27?

Servicing cannot guarantee that an F27 fault will never occur. Electronic components, gas valves and electrodes can still fail. However, routine maintenance provides an opportunity for an engineer to assess the boiler and identify developing leaks, moisture, deterioration, combustion problems, component damage and other conditions requiring attention. Regular maintenance is particularly valuable if the boiler has previously suffered water ingress or repeated electronic faults.