Daikin U0 fault codewhat it means and how to fix it
Shortage of refrigerant in the Altherma outdoor unit. Daikin’s own checklist ends with a leak test, the circuit is sealed, and on an R32 unit this is F-Gas certified work by law.
Take care
Get an F-Gas certified heat pump engineer to look at your Daikin U0
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
Which Daikin appliance is showing U0?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
An Altherma heat pump with radiators or a hot water cylinder
This page is yours. The refrigerant circuit is in the outdoor unit and the fix is a leak repair by a certified engineer.
Note whether the code returns after clearing, then book an F-Gas certified heat pump engineer.
A wall-mounted air conditioner with a remote control
Same two characters, different appliance. Daikin publish U0 as refrigerant shortage for their splits too, but the system and the checks differ.
Use the Daikin U0 page in the air-conditioning section of this site instead.
What U0 means
U0 is published in Daikin’s Altherma error table as "OU: Shortage of refrigerant", and the OU prefix tells you where it lives: in the outdoor unit, on the refrigerant side, which is the part of a heat pump nobody without F-Gas certification may open. This page is for the Altherma heat pump. Daikin air conditioners use the same two characters for the same headline meaning, but the system around them is different and our air-conditioning Daikin page covers that appliance. The service manual for the Altherma 3 M explains what triggers it — refrigerant shortage detected — and what happens: the unit stops, and the code resets automatically or through a power reset at the outdoor unit. That automatic reset is a trap. A unit that clears U0, runs for a while and shows it again is not recovering; it is a compressor being repeatedly asked to work on too little refrigerant, which is how a leak becomes a compressor.
Daikin’s nine-step checklist for an engineer is instructive because of its order: refrigerant-side thermistors and the pressure sensor first, in case the shortage is a false reading; then stop valves, blockages, the charge itself, non-condensables or moisture in the circuit, the compressor and expansion valve; and finally leaks, where Daikin tell the engineer to look for oil traces on the units and at the brazing points on the field piping and to perform a pressure test.
Two things follow for you. A genuine shortage means the refrigerant went somewhere, so the fix is finding the leak and repairing it before recharging, never a top-up. And Altherma 3 units are charged with R32, which Daikin label mildly flammable and which is odourless, so the leak gives no warning of its own. On the North American Altherma 3 H HT W the code also comes with a mode tag: Daikin’s 2026 service manual for that range lists U0-13 as shortage of refrigerant detected in heating mode and U0-14 as the same shortage detected in cooling mode, so a display reading U0-14 or "U014" is this fault, not a different one. The European 3 H HT service manual lists plain U0-00.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Refrigerant has leaked | Daikin’s final check: oil traces on the units, the brazing points on the field piping, and a pressure test. A sealed circuit that is short has a leak. | Common |
| A sensor reporting a shortage that is not there | Daikin put the refrigerant-side thermistors and the pressure sensor at the top of their checklist for exactly this reason. | Occasional |
| Stop valve not fully open or circuit clogged | Both on Daikin’s list. A valve left partly closed at commissioning starves the circuit and looks like a shortage. | Occasional |
| Undercharged at installation | Daikin ask the engineer to check the circuit is correctly charged. A monobloc arrives charged; a split system is charged on site and can be short from day one. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£150–£1,200
Time on site
15 minutes
A diagnostic visit from an F-Gas certified engineer is typically £120 to £250. A leak at an accessible joint, repaired and recharged, sits at the lower end; a coil or a compressor damaged by running short is where the top of the range comes from.
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 Daikin fault codes
- E3Get it checkedThe outdoor unit’s high pressure switch has opened. Daikin list a closed stop valve, a faulty sensor or switch, overcharge, moisture, a blockage and the fan, and require a manual reset.Related: refrigerant circuit
- H4Get it checkedMalfunction of the low pressure switch, a protection device on the refrigerant side of the outdoor unit. Daikin publish the name and nothing an owner can act on; it is certified work.Related: refrigerant circuit
- Bill has jumpedGet it checkedThe most important heat pump fault has no code. If your electricity cost has stepped up with no change in habits, the backup heater is probably doing the work.Related: same brand
- Ice on the outdoor unitGet it checkedNormal in cold damp weather, and the unit clears it itself. Daikin specifically forbid helping — no hot water, no de-icer, no scraping.Related: same brand
- 7HGet it checkedA water flow problem. The sub-code matters more than the code: 7H-01 clears itself, 7H-04 to 7H-06 need a manual reset, 7H-08 is the pump, and 7H-07 is the unit freeing a stuck pump.Related: same brand
- Water at the tap is too coldGet it checkedDaikin give two causes and a remedy that costs money. Before using it, check what your schedule is actually asking the cylinder to do.Related: same brand
Own a Daikin heat pump? Save it and its codes come first next time.
Related guides
Common questions about Daikin U0
Can it just be topped up?
No. The circuit is sealed, so a shortage means a leak, and Daikin’s own procedure ends with finding it: oil traces, brazing points, a pressure test. Recharging without a repair puts refrigerant back into the atmosphere and the fault back on your display a few weeks later.
I have a Daikin air conditioner, not a heat pump. Is this the same U0?
Same headline — Daikin publish refrigerant shortage for both — but this page is written around the Altherma’s outdoor unit, water circuit and R32 charge. Our air-conditioning section has a Daikin U0 page for wall splits, and that is the one to read.
Why is the house still warm?
Because the backup heater has probably taken over. Daikin publish that when the backup heater carries the load, electricity consumption will be considerably higher, so a warm house with U0 showing is a house being heated by direct electric resistance. Get it looked at before the bill arrives rather than after.
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.