Ideal F0 fault codewhat it means and how to fix it
On 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.
Take care
Get a Gas Safe registered engineer to look at your Ideal F0
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 F0 means
Ideal's table for the two-window Logic boards has two rows that sound alike and are not. L3 is No Water Flow — the board cannot see water moving. F0 is No Water Flow Thermistor — the sensor the board uses to make that judgement has itself failed the test. Ideal print the row only in the guides for the Logic Code Combi ES and the first-issue i-mini, both of which carry the two small windows labelled status and burner, and their FAQ lists it as "Flashing F AND 0 No Water Flow Thermistor".
The action they give the householder is the standard one for a sensor: reset the appliance, and if it fails to operate contact Ideal under warranty or a Gas Safe registered engineer outside it. Ideal publish no cause list and no engineer flowchart for F0 in the documents I could find, which is worth knowing when somebody quotes you a confident diagnosis. What the name tells you is enough to steer the visit: it is a sensor and its wiring, not a pump.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor connection loose at the board | The low-voltage connectors on these boards sit at the front of the PCB where the control box cover clips over them, and a plug half out produces an open circuit the board reads as a failed sensor. | Common |
| Thermistor out of range | A clip-on thermistor that has drifted or gone open is the same failure as F4 or F5 on a different pipe, and is dealt with the same way. | Common |
| Board fault presenting as a sensor fault | When the sensor measures correctly and has continuity, the reading is being lost inside the PCB. | 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, if the chart ends there, £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
- 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
- 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 F0
What is a no-water-flow thermistor?
Ideal do not expand the name in the user guide. On these boards the board infers flow from temperature — a heat exchanger that is firing with no water moving through it warms very fast — and the thermistor named here is the sensor that reading depends on. The Logic Max Heat and System of later years replaced the inference with a proper flow sensor, which is why they show Fd instead.
Is F0 the same fault as L3?
No. L3 is the board concluding there is no water flow; F0 is the board saying it cannot trust the sensor it would use to conclude that. One is a circulation problem, the other a sensing one, and they want different parts.
Could this be F0C on a newer boiler?
If your display has three characters and reads F 0C, that is the Logic Combi2's hot water thermistor code and has its own page. F0 with two characters and two windows is this page.
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.