Ideal FA fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Ideal FA
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 FA means
Ideal's table names FA as "Negative Differential Flow/Return Thermistor". The board expects water to leave the heat exchanger hotter than it comes back, and it uses that difference to judge circulation. FA is the difference going the wrong way: the return thermistor is reporting a higher temperature than the flow thermistor.
Since water does not gain heat in the radiators, one of three things is true — a sensor is on the wrong pipe, a sensor is reading wrongly, or the boiler's flow and return have been connected back to front so that water is genuinely going round in reverse. The Vogue manual's name for the same fault, Flow/Return Reversed, points at that last possibility and its checks are the first two: is the flow thermistor clipped to the right pipe, and does it need replacing. It is more often seen after work on the boiler than out of the blue.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Thermistors swapped or clipped to the wrong pipes | The flow and return sensors are the same part on many Logic models. After a repair it is easy to refit them the wrong way round, and the board will faithfully report the world upside down. | Common |
| One sensor drifted | A flow thermistor reading low or a return thermistor reading high produces the same negative gap without anything being physically reversed. | Common |
| Flow and return pipework crossed at installation | Rare on a boiler that has run for years, but a new installation or a system re-pipe that shows FA from the first firing has probably connected the boiler backwards. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£160
Time on site
40 minutes
Re-clipping a sensor is a short visit, £60–£90. A thermistor fitted £80–£160. Re-piping a boiler connected backwards is a different conversation with whoever installed 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
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
- 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 FA
Does FA stop the boiler?
Ideal list it as an F code with the action "if the boiler fails to operate then please contact Ideal or a Gas Safe registered engineer", which is their way of saying it may or may not. In practice the board tends to stop the burner when it cannot trust its circulation reading, so expect lukewarm radiators at best.
Which Ideal ranges show FA?
The Logic Combi from 2011, the Logic ESP1, the Logic Combi2 and the i-mini all print FA; the Independent System guide leaves it out. Ideal's fault codes FAQ lists "Flashing F AND A — Flow & Return Reversed". Vogue Gen2 and Vogue Max show Flow/Return Reversed on the screen.
Is this related to FU?
Same two sensors, opposite complaint. FU is the difference being too large — more than 50°C between flow and return, which points at water not moving. FA is the difference being negative. An engineer will check both thermistors for either code.
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.