Ideal F5 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Ideal F5
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 F5 means
The return thermistor is the second of the pair of clip-on sensors Ideal use to watch the water. Flow tells the board how hot the water is leaving; return tells it how much heat the radiators took out. The gap between the two is how the boiler judges circulation, so a return sensor that lies produces knock-on codes as well as its own — an FA if the two read backwards, an FU if the gap looks impossibly wide.
F5 is the board saying the return reading itself is not credible: open, shorted, or outside the same 9,700 to 10,300 ohms at 25°C that the flow sensor is held to. Ideal's chart for it is short. Pull the sensor off the return pipe, measure it, check continuity to the board, and fit a new thermistor or a new PCB depending on which fails.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor slipped off the pipe or lost its clip | A return thermistor hanging in air reads room temperature while the flow reads eighty degrees, which the board interprets as a fault in the sensor rather than in the clip. | Common |
| Thermistor failed or drifted | The resistances Ideal publish are the test. A sensor that reads far outside them at a known temperature has finished, and it is one of the cheapest parts on the appliance. | Common |
| Broken wire between sensor and board | Ideal's chart asks specifically for continuity between the PCB and the thermistor before anything is replaced; a chafed harness gives an intermittent F5 that vanishes when the case is opened. | Occasional |
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
Thermistor fitted £80–£160. If the chart ends at the PCB, £250–£320 on a Logic.
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
- F4Get it checkedThe 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.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 F5
My display says L5, not F5. Is that this fault?
On the 2011 Logic, the Logic ESP1 and the Logic Combi2 it is: those tables print F5 and L5 together as Return Thermistor. On the 2012–2015 Logic ES boards with two small windows labelled status and burner, L5 means something else entirely — five resets in fifteen minutes — and has its own page here. Check which display you have before reading on.
Why does a return sensor stop the boiler at all?
Because Ideal use the flow-to-return difference to detect a pump that is not moving water. Without a return reading the board cannot make that judgement, and running a heat exchanger with no proof of circulation is the one thing it is designed never to do.
Is this the same as F4?
Same kind of part, different pipe. F4 is the flow thermistor at the top of the heat exchanger; F5 is the return thermistor on the pipe coming back in. They are interchangeable parts on many Logic models, which is why an engineer will sometimes swap them to prove which one is at fault.
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.