What Does the Vaillant F27 Fault Code Mean?
If your Vaillant boiler is showing F27, it means the boiler is detecting a flame when there shouldn't be one. This is called "false flame detection" or "flame simulation."
The boiler locks out as a safety precaution because detecting flame at the wrong time could indicate a gas valve problem. Here's what causes it and when to call us.
Short on time?
The quickest and safest way to resolve an F27 fault is to speak to a qualified Gas Safe engineer. We offer no-obligation phone consultations and quotes and cover North London, West London, Watford, Herts and Middx and nearby.
Key Takeaways
- F27 means false flame detection — the boiler thinks there is flame when there shouldn't be, and locks out as a safety measure.
- The most common causes are a dirty flame sensing electrode or a gas valve not closing properly.
- If you smell gas, leave immediately and call the National Gas Emergency line on 0800 111 999.
- Do not attempt internal repairs yourself — F27 involves gas valve and flame sensing systems that must only be worked on by a Gas Safe engineer.
- Annual servicing prevents most F27 faults by catching electrode and valve problems early.
Quick Checks You Can Do
Try resetting once
Press the reset button and hold for 3 seconds. If F27 returns immediately, don't keep resetting. You need an engineer.
Check for recent power cuts
Sometimes F27 appears after a power interruption. A single reset often clears it if this is the cause.
Listen to the boiler
When it tries to start, listen for any unusual sounds. Note whether you can see flame through the viewing window when F27 appears.
Check the display
Make sure it's definitely F27 and not F72 or F22, which are different faults with different causes.
What Causes Vaillant F27?
The F27 code is specifically about the flame sensing system detecting flame at the wrong point in the ignition sequence. Here are the most common causes:
Faulty flame sensing electrode
The electrode that detects flame presence can become dirty, corroded, or fail completely. If it sends incorrect signals to the PCB, the boiler thinks there's flame when there isn't.
Gas valve not closing properly
This is the most serious potential cause. If the gas valve doesn't close fully when it should, small amounts of gas can leak through and ignite, causing flame detection at the wrong time.
PCB fault
The printed circuit board interprets signals from the flame sensor. If the PCB has a fault, it may incorrectly register flame presence. This is more common on older boilers.
Wiring issues
Damaged or corroded wiring between the flame sensor and PCB can cause false signals. Moisture ingress is a common cause of wiring problems.
Ionisation probe positioned incorrectly
If the probe has been knocked or moved during previous work, it might be detecting residual heat or positioned wrongly.
Residual flame
In rare cases, a small flame can persist after the gas valve closes due to debris or a sticking valve. This is a genuine detection, not a false one, and needs urgent attention.
F27 vs Other Flame Codes
- F27 — Flame detected when there shouldn't be one (false flame)
- F28 — No flame detected at ignition (won't light)
- F29 — Flame lost during operation (lights then goes out)
F27 is the opposite problem to F28. With F28, the boiler can't detect flame. With F27, it's detecting flame when it shouldn't be.
Is F27 Dangerous?
F27 itself is a safety lockout — the boiler has shut down because something isn't right. The lockout prevents any dangerous situation.
However, if F27 is caused by a gas valve not closing properly, this needs prompt attention. A sticking gas valve can allow gas to pass when it shouldn't.
If you smell gas at any time, leave your property immediately and call the National Gas Emergency line on 0800 111 999.
Can I Fix F27 Myself?
There's very little you can safely do with F27:
- Reset once to see if it clears
- Check for obvious external issues (recent power cut, etc.)
Don't attempt to access the boiler internals or touch any components. F27 relates to gas valve and flame sensing systems, which must only be worked on by a Gas Safe registered engineer.
When to Call an Engineer
Call us if:
- F27 returns after reset
- F27 appears repeatedly over several days
- You notice any smell of gas
- The boiler makes unusual sounds when trying to fire
- F27 appeared after other work was done on the boiler
How Engineers Fix F27
Diagnosis first
We test the flame sensing electrode, check gas valve operation, and inspect wiring. The cause needs identifying before any repair.
Common repairs
- Clean or replace flame sensing electrode
- Replace gas valve if not sealing properly
- Repair or replace damaged wiring
- Replace PCB if fault is electronic
- Reposition ionisation probe if moved
Most F27 faults are resolved by electrode cleaning or replacement. Gas valve issues are less common but more expensive to fix.
Preventing F27
Annual service
Your engineer checks flame sensing components and gas valve operation as standard during an annual boiler service. Problems are caught before they cause lockouts.
Don't ignore intermittent F27
If F27 appears occasionally but clears on reset, it's still indicating a developing problem. Better to investigate early.
Keep boiler area ventilated
Good airflow helps prevent moisture buildup that can affect electrical components.
Frequently Asked Questions
Can I fix F27 myself?
You can try a single reset and check for recent power cuts, but F27 involves gas valve and flame sensing components that must only be worked on by a Gas Safe registered engineer. Do not attempt to access the boiler internals.
How much does it cost to fix a Vaillant F27 fault?
Electrode cleaning or replacement typically costs £60–£150. Gas valve replacement is more expensive at £200–£400. At Advanced Heating & Plumbing, we always quote the price before we start. Get a free quote.
Is the F27 fault code dangerous?
The F27 lockout itself is a safety feature — the boiler shuts down to prevent any risk. However, if the cause is a gas valve not closing properly, this needs prompt attention. If you smell gas, leave immediately and call 0800 111 999.
Why does F27 keep coming back after I reset?
A recurring F27 means the underlying cause hasn't been resolved. Most commonly it's a deteriorating flame sensing electrode or an intermittently sticking gas valve. These need professional diagnosis and repair.
What Vaillant models show the F27 code?
F27 can appear on most Vaillant boilers including the ecoTEC Plus, ecoTEC Pro, ecoTEC Exclusive, ecoFIT Pure, and ecoFIT Sustain ranges. The diagnostic process is the same across all models.
Related Vaillant Fault Codes
If you're troubleshooting your Vaillant boiler, these related guides may also help:
- Vaillant F28 Fault Code — No ignition. The boiler tried to light but couldn't detect a flame at all.
- Vaillant F29 Fault Code — Flame lost during operation. The boiler lit but the flame went out.
- Vaillant F22 Fault Code — Low water pressure detected. The boiler shuts down because the system doesn't have enough water.
- Vaillant F75 Fault Code — Pressure sensor or pump fault during start-up.
- Vaillant F20 Fault Code — Overheating protection. The boiler has detected an unsafe temperature.
