Ideal Flow/Return Thermistor Fault
A Vogue Gen2 or Vogue Max message naming both temperature sensors at once. Ideal's checks are whether each thermistor is clipped securely to its pipe and whether both read the right resistance.
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What this means
The Vogue Gen2 and Max have three thermistor rows in their fault menu where a Logic has two. Flow Thermistor Fault and Return Thermistor Fault each point at one sensor; this third message names the pair, and Ideal's remedy for it is different in kind from the others.
Rather than a resistance test on a single named sensor, the chart says: first, check that the flow and return thermistors are securely clipped to their pipes; second, check both against the values in the flow thermistor section — 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. The sequence gives the mechanism away. Both sensors on a Vogue are clip-on NTCs that read the pipe through contact, and one that has slipped off its pipe reads the air in the casing, which lags and drifts as the boiler warms.
The board sees two readings that cannot both be true of the same water and, unable to say which is lying, blames the pair. Ideal do not publish the exact test that trips it, and this page will not guess; what they do publish is that it is a clip check before it is a parts job.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Thermistor clip loose or sensor off its pipe | Ideal's first check, and the reason this message exists separately from the single-sensor ones. A sensor reading casing air instead of pipe metal produces a pair of temperatures that make no sense together. | Common |
| One sensor drifted out of tolerance | An NTC that reads a few kilohms high at 25°C still reports a plausible temperature, just the wrong one. Only comparing the two exposes it, which is what this row is for. | Occasional |
| Wiring to one sensor intermittent | A high-resistance joint in the harness adds ohms to one reading and not the other. Seen as a message that comes and goes with vibration or temperature. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£70–£160
Time on site
30 minutes
A clip re-seated is a call-out, £70–£100. A replacement thermistor on a Vogue is £80–£160 fitted; both together at the top of the range.
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
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
- 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
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions
Is this the same as Flow/Return Reversed?
No. Flow/Return Reversed means the return is reading hotter than the flow, and Ideal's remedy is aimed at the flow thermistor. This message means the two readings are inconsistent in some other way, and both sensors are checked.
Can I keep using the boiler?
Not usefully. The board controls the burner from these two readings, and while it cannot trust them it will not run normally. It is not a gas safety message, but it is a stoppage until the sensors are checked.
Which sensor do I need?
Possibly neither. Ideal put the clip check first because a sensor that has slipped off its pipe needs re-seating, not replacing. If a part is needed, the resistance test says which one.
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.