Ideal L9 fault codewhat it means and how to fix it
On a Logic, L9 is the locked-out form of the unconfigured-board fault F9. On an Isar HE it is something else entirely: a heat exchanger overheat read by the dry-fire thermistor.
Take care
Get a Gas Safe registered engineer to look at your Ideal L9
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.
Which boiler is showing L9?
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.
A Logic, Independent or i-mini with an LCD
Unconfigured or faulty PCB, locked out. The board has to be programmed or replaced, and Ideal treat the size setting as a safety-critical step.
Gas Safe engineer; ring whoever last worked on it if the PCB was recently changed.
An Isar HE with a flashing LED window
Heat exchanger overheat from the dry-fire thermistor. Let it cool; the boiler may show three flat lines until it does. If it recurs the circulation needs looking at.
Engineer if it comes back after cooling.
What L9 means
This is one of the Ideal codes whose meaning depends on which decade your boiler comes from. On a Logic, Independent or i-mini with an LCD display, L9 sits in the same row of Ideal's table as F9 — Unconfigured PCB — and is that fault as a lockout: the board does not know what boiler it is in, or has failed, and has stopped. Everything on the F9 page applies.
On an Isar HE, with its single LED window flashing L and then 9, Ideal's fault-finding chart sends L9 and H9 to the same frame as H4, the dry-fire thermistor: a sensor on the heat exchanger body that reads the metal rather than the water, and L9 is that sensor reporting the heat exchanger itself has run too hot. The Isar manual adds that with the later V9 control board, an L9 overheat that has been reset shows three horizontal lines and will not try to fire until the exchanger has cooled. A reader with a Logic wants a board; a reader with an Isar wants a pump, a flush or a thermistor.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Logic: board never configured or configuration lost | The lockout form of F9. Common after a PCB change that was not finished, occasional on an undisturbed boiler whose board has failed. | Common |
| Isar: poor circulation through the heat exchanger | A tired pump, a sludged exchanger or closed valves let the metal outrun the water. Ideal's dry-fire frame starts by asking whether the system is filled, vented and open. | Common |
| Isar: dry-fire thermistor or its wiring | The same frame checks continuity from PCB to sensor and the sensor's resistance before replacing either. A sensor that has drifted high reports an overheat that did not happen. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£380
Time on site
1 hour
Logic: configuring a board £60–£90, replacing one £250–£380. Isar: a dry-fire thermistor £90–£160 fitted; a pump £180–£300 if circulation is the cause.
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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Ideal Logic primary PCB
£150–£320Authorised 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
- F9Get it checkedThe control board has not been told what boiler it is in, or has failed. L9 is the same fault locked out. The 2011 Logic Combi row also covers a gas valve short circuit.Related: electronics and power
- dUUsually simple to fixNot a fault. The diverter valve has been parked in its mid-position for servicing and the boiler is waiting to be told the service is over. Ideal publish the way out.Related: electronics and power
- Blank displayGet it checkedNothing on the screen at all. Ideal's user guides send you to the boiler's switch and the consumer unit first; the installer charts then work from the terminal block to the board.Related: electronics and power
- F7Get it checkedThe mains voltage reaching the boiler has dropped below what the board will run on. Ideal's advice is an electrician or your electricity supplier, not a heating engineer.Related: electronics and power
- C0Get it checkedThe board cannot activate the Boiler Chip Card that tells it what model it is fitted to. Ideal's engineer check is whether the small plastic card is in its slot at all.Related: electronics and power
- SEUsually simple to fixA service reminder, not a fault. The installer set a timer at the last visit and it has run down. Ideal's advice is to contact the installer, who resets it when the boiler is serviced.Related: electronics and power
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
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.