Ideal L6 Fault Code: Causes, Fixes & Repair Costs
What does the Ideal L6 fault code mean?
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.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or contaminated flame detection electrode Common
The flame sensing electrode sits inside the combustion chamber and passes a small ionisation current through the burner flame to confirm ignition. If the electrode tip is coated in carbon deposits, has deteriorated with age, or was disturbed during a recent service, it can send a spurious signal to the PCB indicating a flame is present even when the burner is off. This is the single most frequently diagnosed cause of L6 on Ideal boilers.
- Sticking or leaking gas valve Common
The gas valve is designed to seal completely when there is no call for heat. If internal valve components wear or stick, the valve can leak a small quantity of gas into the burner even when it should be fully closed — a condition known as 'letting-by'. The flame electrode then correctly detects residual combustion, and the boiler locks out. A partially open gas valve is a safety-critical issue and must be assessed by a Gas Safe engineer.
- PCB fault or software glitch Sometimes
The printed circuit board interprets the electrical signal from the flame electrode. A failing PCB can misread this signal and report a flame where none exists, or it may fail to correctly command the gas valve to close. Occasionally a temporary electronic glitch rather than a component failure is responsible, which is why a single power cycle is worth attempting before calling an engineer.
- Wiring or connection fault Sometimes
Damaged, corroded, or loose wiring between the flame electrode and the PCB can introduce stray voltages that mimic a genuine flame signal. Moisture ingress near connectors is a known contributor on older installations. An engineer will inspect the full electrode circuit as part of diagnosing L6.
- Low or unstable gas supply pressure Rare
If the gas pressure arriving at the boiler is erratic, the burner may behave unpredictably — producing intermittent flame events that the electrode picks up outside of a normal heat demand cycle. This is less common but worth the engineer checking, particularly if other gas appliances in the property are also behaving oddly.
How to fix it
- Check that your gas supply is working DIY safe
Before anything else, confirm that other gas appliances in your home — such as a gas hob or fire — are functioning normally. If nothing gas-powered works, contact your gas supplier, as there may be a supply interruption in your area. Do not attempt to investigate the boiler's gas pipework yourself.
- Power cycle the boiler once DIY safe
Switch the boiler off at the fused spur on the wall, wait a full 30 seconds, then switch it back on. This clears any temporary electronic glitch that might have triggered a false flame reading. Watch the boiler carefully through its startup sequence.
- Attempt a single boiler reset DIY safe
If the boiler is still showing L6 after the power cycle, press and hold the reset button (or turn the dial to the reset position — refer to your Ideal model's manual for the exact method). Allow the boiler to complete one full ignition attempt. If it fires and runs normally without the code returning within the hour, monitor it closely over the next day or two. If the L6 code reappears, do not reset again — move to the next step.
- Do not reset the boiler repeatedly DIY safe
Resetting more than once or twice does nothing to fix a flame sensing or gas valve fault, and could mask a safety-relevant issue. If L6 returns after your single reset, switch the boiler off at the fused spur and leave it off until a Gas Safe engineer has inspected it.
- Have a Gas Safe engineer inspect the flame detection electrode and circuit Gas Safe engineer
An engineer will remove and inspect the flame sensing electrode, check its positioning in the combustion chamber, test the ionisation signal strength, and clean or replace the electrode as needed. They will also check the wiring loom and connectors throughout the flame detection circuit for signs of damage or corrosion.
- Have a Gas Safe engineer test and assess the gas valve Gas Safe engineer
The engineer will check whether the gas valve is sealing correctly when closed. A valve that is letting-by will need to be replaced — this is gas-side work that is illegal for an unregistered person to carry out. Parts and labour for a gas valve replacement typically cost more than a sensor swap, so confirming the diagnosis first is important.
- Have the PCB tested if other components check out Gas Safe engineer
If the electrode, wiring, and gas valve are all found to be in good order, the PCB becomes the likely culprit. The engineer may be able to test the board's flame signal processing, or may recommend replacement. Given the cost of a new PCB, it is worth discussing the age and overall condition of the boiler before authorising this repair on an older unit.
- Call a Gas Safe registered engineer Gas Safe engineer
If the single reset has not cleared the fault, this code requires professional diagnosis. You can find a Gas Safe registered engineer in your area at gassaferegister.co.uk. Provide them with the exact fault code (L6) and your boiler model so they can bring likely spare parts on the first visit.
Parts you may need
- Flame detection electrode · from £25
- Electrode lead/ignition lead set · from £35
- Gas valve (Ideal-compatible replacement) · from £130
- PCB (model-specific) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£350, depending on the underlying cause.
Frequently asked questions
Is the Ideal L6 fault code dangerous?
It warrants prompt attention rather than panic. The boiler has detected a flame signal when the burner should be off and has locked out precisely to prevent an unsafe situation from developing. The most common culprit — a dirty or faulty flame electrode — is not inherently dangerous, but a sticking gas valve that is letting gas through when it should be closed is a more serious matter. Either way, repeated resetting is not appropriate: switch the boiler off and arrange for a Gas Safe engineer to diagnose it.
Can I fix the Ideal L6 fault myself?
The only checks a homeowner should attempt are confirming the gas supply is on, doing one power cycle, and trying a single boiler reset. Everything beyond that — inspecting or replacing the flame electrode, testing the gas valve, or working on the PCB — must be done by a Gas Safe registered engineer. Gas work carried out by an unregistered person is illegal in the UK and could invalidate your home insurance.
How much does it cost to fix an Ideal boiler L6 fault?
Most people pay somewhere between £150 and £350 all-in, covering the engineer's call-out, labour, and the replacement part. A straightforward electrode clean or swap sits toward the lower end of that range, while a gas valve replacement typically pushes toward the higher end. PCB replacement is less common as the root cause of L6, but if required it can push the total cost above £400 on some Ideal models — at which point it is worth factoring in the boiler's age when deciding whether to repair or replace.
Will an annual boiler service help prevent the L6 fault?
Yes, in many cases. A thorough annual service includes inspecting and cleaning the flame detection electrode, checking the electrode's positioning and connections, and testing gas valve operation — all of which can catch early signs of deterioration before they develop into a lockout. Keeping up with yearly services is the single most effective way to reduce the likelihood of fault codes like L6 appearing out of the blue.