Ideal H 2 fault codewhat it means and how to fix it
The return thermistor on an Isar HE is reading outside its range. It is the temporary partner of H 1 and Ideal test it the same way; it is not listed on the heat-only Icos.
Take care
Get a Gas Safe registered engineer to look at your Ideal H 2
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What H 2 means
H 2 is Control Thermistor (Return) Error, the second sensor on an Isar reporting a reading the board will not accept. The return thermistor sits on the pipe bringing water back from the radiators and gives the board the other half of the flow-to-return differential it uses to judge circulation — the 25°C figure that appears in the overheat chart is measured between this sensor and the flow one.
Ideal's frame for H 2 is the H 1 frame with the pipe changed: remove the return thermistor from the CH return pipe, disconnect it, check the resistance against the same three-temperature table, replace the sensor if the values are wrong; if they are right, check continuity to the board, repair the wiring or replace the PCB. It is an Isar code specifically. The Icos M3080 installer manual's fault menu has no H 2, because that boiler's later board does not monitor a return sensor in the same way, and the Mexico HE and Esprit share the Isar table. On a Logic the same sensor and the same test produce F5.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Return thermistor drifted | Decided by Ideal's resistance test. The return sensor runs cooler than the flow one and fails less often, but it is the same component and the same age. | Common |
| Sensor off its pipe | A clip-on sensor that has slipped reads the casing air, and a return that appears to be at room temperature while the flow is at 70°C is an impossible differential. | Occasional |
| Wiring or board | The two boxes after the resistance test: continuity from the board to the sensor, then the PCB itself. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£380
Time on site
40 minutes
A thermistor £80–£150 fitted; wiring £70–£110; a PCB £200–£380 if the chart runs to its end.
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
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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 H 2
How is H 2 different from H 1?
The pipe. H 1 is the sensor on the flow pipe out of the heat exchanger, H 2 the sensor on the return pipe back to it. The board needs both to judge how much heat the system is taking, and either one out of range stops it.
Why does my Icos never show H 2?
Because the Icos installer manual does not list it. Its fault menu runs L F, L E, L A, L 8, H 1, H F, H 4, H n and H E. If an Icos is showing something that looks like H 2, the code is worth reading again against that list.
Will the overheat trip because of this?
Not directly — the overheat protection reads the flow side. But a return sensor reading wrongly upsets the differential the board uses, and Ideal's L A chart uses that same differential as its pump test, so an engineer chasing an overheat may find H 2 behind it.
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.