Ideal F0C fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Ideal F0C
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 F0C means
F 0C is a code the 2023 three-character Logic boards added. Earlier Logic combis had no separate fault for the domestic hot water thermistor; the Combi2 and i-mini2 do, and Ideal's manual gives it a section of its own. The sensor sits on the hot water outlet from the plate heat exchanger and tells the board how hot the water going to the tap actually is, so the board can modulate the burner to the temperature on the dial.
When it reads outside Ideal's expected values — 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 at 60°C, the same thermistor family as the flow and return — the board reports F 0C. The engineer chart is the familiar one: disconnect the sensor, measure it, fit a new one if it is wrong, check continuity to the PCB if it is right, replace the board as the last resort. On a Logic+ or Logic Max of the same year the screen says DHW Thermistor Fault with Contact Installer beneath it.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor drifted or failed | The hot water thermistor lives on the outlet pipe and sees the fastest temperature swings in the boiler, from cold to sixty degrees every time a tap opens. They do wear. | Common |
| Plug or wiring to the PCB | Ideal's chart checks continuity between the thermistor and the board before the board is blamed, because a harness pinched under the control box cover fails intermittently. | Common |
| PCB input fault | Rare and last on the chart: a sensor that measures correctly with good wiring and a board that still reports it. | 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 thermistor fitted £80–£160; a PCB £250–£320 if the chart reaches it.
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
- 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 about Ideal F0C
Will I still have heating?
Usually. Ideal list it as an F code, and the sensor is only consulted when hot water is being drawn. Expect the heating to run and the hot water to be unreliable until it is fixed.
Which boilers show F0C?
The Logic Combi2 and i-mini2 C, both from 2023, whose displays have three characters. The Logic+ Combi2 and Logic Max Combi2 show DHW Thermistor Fault in words. No earlier Logic has a code for this sensor, and a Logic System or Heat has no such sensor at all.
Is this the same as FC on an older boiler?
There is no FC in any Ideal table read for these pages. If an older Logic shows something that looks like FC, look again for the L — L C, five resets in fifteen minutes, is the code that exists.
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.