L6 Boiler Fault Code: What It Means on Every Brand

The L6 fault code appears on 2 boiler brands — Ideal and Keston — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 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 L6 means	Severity	DIY?
IIdeal	The L6 code appears on Ideal boilers when the boiler's flame detection electrode registers a flame signal at a moment when no heating or hot water has been requested — meaning the gas… Full guide →	Lockout — engineer likely	No
KKeston	The L6 code on a Keston boiler indicates a false flame lockout. Full guide →	Lockout — engineer likely	No

L6 brand by brand

Ideal L6

The L6 code appears on Ideal boilers when the boiler's flame detection electrode registers a flame signal at a moment when no heating or hot water has been requested — meaning the gas valve should be fully closed and no combustion should be occurring. Because this situation could indicate gas reaching the burner unexpectedly, the boiler locks out immediately as a safety precaution. This is sometimes called a 'false flame lockout', though the word 'false' refers to the timing of the signal rather than implying the reading is definitely wrong. The boiler should be treated as a priority fault: do not repeatedly reset it, as the underlying cause needs professional diagnosis.

Causes, fixes & costs for Ideal L6 → · All Ideal codes

Keston L6

The L6 code on a Keston boiler indicates a false flame lockout. This means the boiler's control system has detected a flame signal at a point in the firing cycle when no flame should be present — or it has received an erratic, unstable flame signal that doesn't match expected behaviour. Because an unexpected flame signal represents a genuine safety risk, the boiler shuts itself down and locks out, refusing to restart until the reset button is pressed. Simply switching the boiler off and back on at the mains will not clear the lockout — you must use the dedicated reset button on the boiler's control panel.

Causes, fixes & costs for Keston L6 → · All Keston codes

Frequently asked questions

Is the L6 fault code the same on every boiler?

Broadly, yes — on Ideal and Keston the L6 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 L6 fault myself?

Not usually. On every brand listed, L6 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 L6 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.