Ideal F4 fault codewhat it means and how to fix it
The flow thermistor — the sensor reading the temperature of water leaving the heat exchanger — is giving a value the board will not accept. L4 is the same fault as a lockout.
Take care
Get a Gas Safe registered engineer to look at your Ideal F4
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 letter is in front of the 4?
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.
F4 — the boiler may still run
The board has seen a bad reading but has not locked out. Expect it to become L4 if the reading stays bad, and expect erratic radiator temperatures in the meantime.
L4 — the boiler has stopped
Locked out on the same sensor. Restart clears it once; if it locks out again the sensor or its wiring has to be looked at.
Book an engineer rather than resetting repeatedly.
What F4 means
Ideal print F4 and L4 in one row of the fault table, headed Flow Thermistor. The thermistor is a small resistor clipped to the flow pipe at the top of the heat exchanger whose resistance falls as the water warms, and the board reads that resistance to decide when to modulate down and when to stop.
A reading outside the range Ideal expect — 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 at 60°C, 1,000 to 1,100 at 85°C — is an open circuit, a short, or a sensor that has drifted, and the boiler stops rather than fire blind. Without a believable flow temperature it cannot protect the heat exchanger from overheating, so the stop is prudence rather than damage. It appears as F while the board is still trying, and becomes L4 once it has stopped trying.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Wiring plug loose at the PCB or the sensor | Ideal's fault-finding chart checks the six-way low-voltage connector at the front left of the PCB and the connection at the thermistor before it lets the engineer condemn anything. | Common |
| Thermistor out of tolerance | The sensor itself has failed or drifted. Ideal publish the resistances to expect at three temperatures, and a meter across the pins settles it in a minute. | Common |
| PCB no longer reading the sensor correctly | When the thermistor measures right and the wiring has continuity, Ideal's chart has one step left, which is to replace the board. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
45 minutes
A flow thermistor is a cheap part and most of the bill is the visit: £80–£160 fitted. A PCB, if that is where the chart ends up, is £250–£320.
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 flow or return thermistor (clip-on NTC)
£12–£35Authorised Ideal parts distributor
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
- F5Get it checkedThe return thermistor on the pipe bringing water back from the radiators is reading outside its range. On most Logic boards L5 is the locked-out form of the same fault.Related: sensors and thermistors
- L5Get it checkedTwo different faults share this code. On the two-window Logic ES boards it means five resets in fifteen minutes; on the 2011 Logic, the ESP1 and the Combi2 it is a return thermistor lockout.Related: sensors and thermistors
- F6Get it checkedThe outside sensor for weather compensation is reading wrongly or is not connected. It only appears on boilers that have one, or have been told they have one.Related: sensors and thermistors
- FAGet it checkedThe return pipe is reading hotter than the flow, which cannot happen in a boiler that is plumbed and sensing correctly. Ideal call it a negative differential; the Vogue screen says Flow/Return Reversed.Related: sensors and thermistors
- F0Get it checkedOn the Logic Code Combi ES and first i-mini, F0 is the sensor that detects water flow reporting a fault. It is not the same as L3, which is the flow itself having stopped.Related: sensors and thermistors
- F0CGet it checkedThe hot water thermistor on a Logic Combi2 or i-mini2 is out of range. Heating usually carries on; the tap side suffers, because the board can no longer see the water it is making.Related: sensors and thermistors
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal F4
Is F4 the same as L4 on an Ideal?
Yes. Ideal's user guides for the Logic, Independent and i-mini put F4 and L4 side by side in one row, and their fault-finding chart heads the section "F4 or L4 — Flow Thermistor Fault". The difference is state, not fault: F is the board reporting, L is the board having stopped.
Can I run the boiler on hot water only until it is fixed?
Not reliably. The flow thermistor is read in both heating and hot-water modes because it protects the heat exchanger in both, so a sensor the board does not trust stops everything. A lockout on this sensor is not something a mode change gets round.
Which Ideal boilers show this code?
Every Logic generation prints it — F4 on the 2011 and ES boards, F 04 on the ESP1, F 04 or L 04 on the Combi2 — and so do the Independent, i-mini and Logic Code Combi. On a Logic+, Logic Max or Vogue the same fault appears in words as Flow Thermistor Fault. The old Isar and Icos report their flow thermistor as H 1 instead.
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
- Manufacturer support portal
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.