What the F27 code means
Every modern boiler checks for a flame using an electrode that sits in the burner. When the gas lights, a tiny electrical current passes through the flame and the circuit board reads it as proof that the burner is alight. That same check also runs the other way. When the boiler has shut the gas off, the board expects to see no flame at all.
On most ecoTEC models, F27 is listed as a safety switch-off for flame simulation. It means the board is reading a flame signal when the gas valve should be closed. Either something is fooling the sensor into seeing a flame that is not there, or gas really is still reaching the burner. The boiler cannot tell which, so it locks out to be safe. Because the fault involves the flame safety system, the boiler will not run at all while F27 is showing, so you will have no heating or hot water until it is sorted.
That makes F27 quite different from ignition codes such as F28, where the boiler cannot find a flame it wants. With F27 it finds one it does not want.
Common causes of F27
Moisture on the electronics
Vaillant's manual lists moisture on the electronics as the first possible cause. Water from a small internal leak, condensation, or a leak from pipework or a bathroom above the boiler cupboard can reach the circuit board and create a stray signal.
A faulty flame detection circuit
The part of the PCB that monitors the flame can fail. When it does, it may report a flame all the time. This is one of the more common reasons an engineer ends up replacing the board.
A gas valve that is letting by
If the gas valve does not close fully, a small amount of gas can keep burning after the boiler has switched off. That is a real flame, and the boiler is right to lock out. The manual lists a leaking gas solenoid valve as a possible cause.
Electrode or ignition lead problems
A damaged electrode, cracked insulation or a worn ignition lead can also cause false readings, according to installer fault lists.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Moisture on the PCB | F27 after damp weather or a visible leak nearby | Gas Safe engineer |
| Faulty flame detection circuit | F27 returns straight after every reset | Gas Safe engineer |
| Gas valve not closing fully | Burner seems to stay lit briefly after switching off | Gas Safe engineer |
| Worn electrode or lead | Intermittent F27, sometimes with ignition faults | Gas Safe engineer |
Safe checks you can do yourself
There is not much a homeowner can safely do with F27, because every likely cause is inside the boiler. These checks help you give the engineer useful information:
- Write down the code and the time it appeared, plus anything unusual, such as heavy rain or someone running a bath upstairs.
- Look (without touching anything or removing panels) for water dripping onto the boiler from pipes, a ceiling or a shelf above.
- Check the area under the boiler for drips or a damp patch. Our guide on a boiler leaking water explains what different leaks usually mean.
- Check the fused spur is on and the power supply is steady.
- Press reset once as your manual describes. If F27 returns, try one more time at most, then leave the boiler off and call an engineer.
If you can smell gas at any point, do not press reset or operate electrical switches. Turn off the gas at the meter if it is safe to do so, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
F27 is a job for a Gas Safe registered engineer in almost every case. Call one if the code returns after a reset, if you have seen water near the boiler, or if the boiler has been showing other faults. The engineer will check for water inside the case, inspect the electrode and lead, test the gas valve to confirm it shuts off fully, and then test the PCB. Replacing the board without checking the gas valve first would be a mistake, which is why a proper diagnosis matters.
Do not remove the casing to dry anything out yourself, and do not keep resetting the boiler to get some heat. The flame check is there to protect you.
Typical repair costs in the UK
The price depends heavily on which part has failed. These are typical ranges only.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Replacing the ignition or detection electrode | Typically £90 to £180 |
| Replacing the gas valve | Typically £200 to £400 |
| Replacing the main PCB | Typically £250 to £500 |
| Fixing a leak above or inside the boiler | Typically £80 to £250 |
If your boiler is still under the manufacturer's warranty, contact Vaillant or your installer before booking anyone else, as the repair may be covered.
How to stop F27 coming back
Keep the boiler dry. Get any drips from pipes, tanks or bathrooms above the boiler cupboard fixed promptly, and do not store damp items against the casing. A yearly service lets an engineer spot corrosion on connectors, a worn electrode or signs of water inside the case before they trigger a lockout. Our boiler service cost guide explains what a service includes. If the gas valve was replaced, ask the engineer to confirm the gas pressure and combustion readings are correct afterwards.