Ideal dU status codewhat it means
Not a fault. The diverter valve has been parked in its mid-position for servicing and the boiler is waiting to be told the service is over. Ideal publish the way out.
Before you start
What dU means
dU is the one entry in Ideal's fault table that is a mode rather than a malfunction. Inside a combi, the diverter valve swings between heating and hot water, and to change it an engineer needs it held halfway. Ideal's installer menu has an option for exactly that — the ESP1 manual lists dU among the installer functions alongside SH for maximum rate and VF for venting — and once selected the boiler sits there displaying dU until it is released.
The code on a householder's display therefore means one of two things: the engineer left without cancelling the service position, or somebody has been through the installer menu who should not have. Ideal print the way out in the user guide, and it is the only fault-table row whose remedy is entirely in the householder's hands: turn all the knobs fully clockwise, switch the boiler off and on, then press Restart. The Combi2 shortens it to "press Restart to end".
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Service position not cancelled after a repair | The engineer replaced the diverter or its motor, tested the boiler, and left the board in the mode that made the job possible. | Common |
| Installer menu entered by accident | The menu is reached by a button combination that is not hard to hit while cleaning the front panel. Stepping through it with Restart pressed at the wrong prompt parks the valve. | Occasional |
What to try yourself
Turn both temperature knobs — hot water and heating — fully clockwise. On a Logic Heat there is only one. This is Ideal's published first step and it matters; the board reads the knob positions on the way back up.
Switch the boiler off at the fused spur or its own switch, wait a few seconds, and switch it back on.
Press Restart once. The display should return to normal and the knobs can go back to where you had them. On a Logic Combi2 Ideal say pressing Restart alone ends the mode.
Get a Gas Safe registered engineer to look at your Ideal dU
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 it costs to fix
Typical repair cost
£0
Time on site
5 minutes
Nothing. It is a reset, published by Ideal for the householder.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Ideal fault codes
- F9Get it checkedThe control board has not been told what boiler it is in, or has failed. L9 is the same fault locked out. The 2011 Logic Combi row also covers a gas valve short circuit.Related: electronics and power
- Blank displayGet it checkedNothing on the screen at all. Ideal's user guides send you to the boiler's switch and the consumer unit first; the installer charts then work from the terminal block to the board.Related: electronics and power
- F7Get it checkedThe mains voltage reaching the boiler has dropped below what the board will run on. Ideal's advice is an electrician or your electricity supplier, not a heating engineer.Related: electronics and power
- C0Get it checkedThe board cannot activate the Boiler Chip Card that tells it what model it is fitted to. Ideal's engineer check is whether the small plastic card is in its slot at all.Related: electronics and power
- L9Get it checkedOn a Logic, L9 is the locked-out form of the unconfigured-board fault F9. On an Isar HE it is something else entirely: a heat exchanger overheat read by the dry-fire thermistor.Related: electronics and power
- SEUsually simple to fixA service reminder, not a fault. The installer set a timer at the last visit and it has run down. Ideal's advice is to contact the installer, who resets it when the boiler is serviced.Related: electronics and power
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal dU
Is anything wrong with the boiler?
Almost certainly not. dU is a service state, listed in the fault table only because it stops the boiler running and a householder would otherwise have no way of knowing why. Ideal give a full reset procedure for it and no instruction to call anyone.
It will not clear when I follow the steps.
Then the diverter valve motor may not be able to move out of mid-position, which is a repair rather than a reset. It is not urgent in the sense of being dangerous, but you will have no heating or hot water until it is done.
Which boilers show dU?
Logic Combi from 2011 onwards, the Logic ESP1, the Logic Combi2 and the i-mini — combis, because only a combi has a diverter valve. Ideal's FAQ spells it "Flashing D AND U". A Logic System or Heat has no such mode.
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.