Ideal L F fault codewhat it means and how to fix it
An Isar, Icos, Mexico HE or Esprit has made three attempts to light and locked out; the LED window flashes L then F. Ideal permit one reset and a gas supply check, then an engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Ideal L F
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What else can you see?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
The hob or gas fire will not light either
The boiler is reporting a supply problem correctly.
Check any prepayment meter and the emergency control valve at the meter, then contact your gas supplier. Do not keep resetting the boiler.
It is freezing outside and it worked yesterday
The condensate pipe has very likely frozen, which Ideal list as a cause of exactly this code.
Thaw the external plastic pipe with warm water from a jug, never boiling, then hold reset for two seconds once.
You have held reset for two seconds and L F came straight back
Three more attempts have failed. The fault is inside the boiler — electrode, spark generator, gas valve or condensate trap.
Stop resetting and book a Gas Safe registered engineer.
You can smell gas
Leave the boiler alone.
Open windows, do not operate switches, leave the property and call 0800 111 999 from outside.
What L F means
On Ideal's LED-display boilers the ignition lockout is L F, shown as a single character window flashing L, then F, then L again, and Ideal's user guide describes the moment exactly: "attempts to fire by going through 3 ignition attempts, display shows L:F". Three is the number, not the five a Logic makes. The installer chart calls it Flame Detection Error and works through the ignition electrode and its lead, then whether the burner lights briefly and dies, then whether gas pressure is at the inlet, whether roughly 200 volts DC is reaching the gas valve during the attempt — a rectified voltage that reads anywhere above 150 on most meters — then the spark generator, and finally the gas valve itself.
The householder's share, printed in the guide, is three lines: hold the reset button for two seconds to repeat the sequence; check that another gas appliance works; check the condensate pipe is not blocked or frozen. Because the code stands for gas admitted three times without a flame, this page gives those as conditions rather than as a list to work down, and stops at one reset.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| No gas at the boiler | Ideal's own second check. A cooker or fire that will not light either means the meter, a prepayment balance or the supply, and nothing at the boiler will change that. | Common |
| Ignition or detection electrode | The first and third boxes in Ideal's chart: continuity, condition and position of the electrode and its harness. These boilers are old enough for a worn electrode to be the ordinary explanation. | Common |
| Condensate trap or pipe blocked | Ideal print this for the householder and the engineer both. A frozen external run stops the burner within seconds of lighting, and the boiler reads it as a failed ignition. | Common |
| Spark generator or gas valve | The last boxes in the chart, reached only once gas pressure and the 200 V DC valve supply have been proved. On a fifteen-year-old Isar the spark generator is a known wear item. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£300
Time on site
20 minutes
Free if the gas supply or a frozen pipe is the cause. An electrode £90–£160 fitted; a spark generator £110–£180; a gas valve £180–£300. On a boiler of this age an engineer may reasonably raise replacement before a gas valve.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Ideal Logic ignition electrode
175565£18–£42Authorised Ideal parts distributor
Price checked 28 September 2026
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Ideal Logic gas valve
£90–£200Authorised Ideal parts distributor
Price checked 28 September 2026
Ideal Logic spark generator
£35–£80Authorised Ideal parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Ideal fault codes
- L2Stop — call an engineerThe boiler failed to detect a flame during ignition and has locked out on the gas valve. Do not keep resetting it.Related: ignition and gas supply
- F2Get it checkedThe flame was lost during operation. Ideal direct you to check the gas supply first, and that is genuinely the right order.Related: ignition and gas supply
- LCUsually simple to fixYou have reset the appliance five times in fifteen minutes. This is not a fault in itself — it is the boiler asking you to stop.Related: ignition and gas supply
- L6Stop — call an engineerA flame is being detected when there should not be one. Turn the appliance off and call a Gas Safe engineer rather than resetting it.Related: ignition and gas supply
- F16Stop — call an engineerThe board has detected a fault in the gas valve or its wiring. Nothing here is for a householder to try; Ideal's checks cover the valve's supply and coil and end with replacing it.Related: ignition and gas supply
- F10Get it checkedThe spark generator on a Logic Combi2 — the unit that makes the ignition spark — has failed a self-check. Ideal's instruction is to check its wiring and, if that is sound, replace it.Related: ignition and gas supply
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal L F
Is L F the same as L2 on a Logic?
It is the same fault on a different display. The Logic shows L2 after five ignition attempts; the Isar, Icos, Mexico HE and Esprit show L F after three. If your display has letters and numbers side by side you have a Logic and want the L2 page instead.
How do I reset an Isar or Mexico HE?
Hold the reset button on the control panel for two seconds and release; the boiler repeats the ignition sequence. On Isar and Esprit models with the built-in programmer, holding the Select button for more than three seconds does the same. Once is the sensible limit for this code.
Why does the Mexico HE guide mention L2 for a frozen condensate pipe?
It is a hangover from Ideal's Logic text. The Mexico HE's LED window cannot show a 2 in that position; its own fault table gives L F, and that is what you will see.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.