What the L6 code means
Your boiler uses a flame sensing electrode to confirm that the burner is lit. When the gas valve opens and the spark fires, the electrode picks up a small electrical signal from the flame. When the burner goes out, that signal should disappear. The control board watches this signal all the time, and it expects the answer to match what it has told the boiler to do.
A false flame is when the board sees a flame signal at the wrong time, for example before the boiler has tried to light, or after the gas valve has been told to close. That does not make sense, so the boiler cannot trust its own flame detection. It locks out with L6 and waits for a manual reset. Ideal lists L6 as a false flame lockout in the fault finding section of its Logic, Logic Combi2 and Logic Max installation manuals.
Because L is a lockout code, L6 will not clear by itself. The boiler stays off until it is reset.
Common causes of L6
A faulty or contaminated flame sensing electrode
The electrode sits in the combustion chamber and slowly builds up deposits. A worn, cracked or badly positioned electrode can give an unreliable signal, including one that suggests a flame is present when it is not.
Moisture or condensation in the combustion area
Condensing boilers produce a lot of water. Damp around the electrode or its lead can create a stray electrical path that the board reads as a flame signal.
A gas valve that is not closing fully
If the gas valve lets a small amount of gas through when it should be shut, a small flame can keep burning after the boiler has turned off. This is the most serious possible cause, and it is one reason L6 should always be looked at by an engineer if it repeats.
Wiring or control board faults
A damaged electrode lead, a poor earth or a fault in the flame detection circuit on the board can all produce a false reading.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Worn or dirty flame sensing electrode | L6 now and then, often with a history of ignition trouble | Gas Safe engineer |
| Moisture near the electrode | Fault in damp or very cold weather | Gas Safe engineer |
| Gas valve not closing fully | L6 soon after the burner shuts down | Gas Safe engineer |
| Wiring, earth or PCB fault | Electrode and valve test fine | Gas Safe engineer |
Safe checks you can do yourself
A false flame fault involves gas and flame detection parts, so your checks are limited. These steps are safe.
- Check for any smell of gas around the boiler or meter. If you smell gas, do not reset the boiler. Follow the warning below.
- Check the fused spur is on and that there have been no recent electrical problems in the house.
- Press reset once, following the method in your manual. On many Ideal models you hold the reset button for a few seconds.
- Watch the boiler for a few minutes. Note whether L6 returns during start-up, while the burner is running or just after it shuts down.
- If it returns, reset no more than two or three times in total. After that, switch the heating off at the controls and call an engineer.
If you smell gas, 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. Never remove the casing or touch the electrode, gas valve or PCB yourself.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if L6 comes back after a reset, or if it appears more than once in a short period. The engineer will inspect and test the flame sensing electrode and its lead, check the gas valve is closing properly, and test the earth and flame detection circuit on the board. They will also run a combustion check.
When you book, tell them the exact code and when it appeared. A false flame shortly after the burner shuts down points towards the gas valve, while a fault on start-up points towards the electrode or board. That detail helps them bring the right parts.
Too many resets in a short time can also lead to an LC lockout, which is another reason not to keep pressing reset. If your boiler is still under its manufacturer's warranty, contact Ideal or your installer first, as electrode and valve faults may be covered.
Typical repair costs in the UK
These are typical ranges including parts and labour. Prices vary by model, area and engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Ignition or flame sensing electrode replacement | Typically £100 to £180 |
| Ignition lead replacement | Typically £80 to £150 |
| Gas valve replacement | Typically £250 to £450 |
| Replacement PCB | Typically £300 to £550 |
How to stop L6 coming back
The flame sensing electrode wears slowly, and a yearly service is the best time to inspect and replace it before it causes trouble. Our boiler service cost guide explains what a service includes. Keep the condensate pipe clear in winter, because water backing up into the boiler can cause a range of faults. Our frozen condensate pipe guide explains how to thaw it safely. If your boiler struggles to light rather than showing a false flame, see our page on the Ideal L2 ignition lockout.