U0-00 on a Daikin Altherma: OU: Shortage of refrigerant
Short answer
On a Daikin heat pump, U0-00 means: OU: Shortage of refrigerant. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: OU: Shortage of refrigerant.
Part involved: refrigerant circuit / pressure protection.
When it shows: Refrigerant shortage detected.
Likely causes:
- Faulty refrigerant side thermistor(s).; Faulty refrigerant pressure sensor.; Closed stop valve in the refrigerant circuit.; Clogged refrigerant circuit.; Refrigerant shortage.; Non‑condensables and/or humidity in the refrigerant circuit.; Faulty compressor or miswiring of the compressor power supply cable.; Faulty expansion valve.; Leak in the refrigerant circuit.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: Automatic reset. Power reset via outdoor unit.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) and note it, with the date/time, for your installer.
- Check the heating-system water pressure reading on the gauge/display: Daikin operation manuals ask users to check it is above 1 bar. If it is below that, report it to your installer (do not open the unit).
- Look at the outdoor unit from outside: check the air inlet/outlet and fan grille are not blocked by leaves, debris, snow or heavy ice build-up (do not remove panels or touch the fan).
- Daikin documents this code as resetting automatically once the condition clears; if it keeps coming back, contact your installer.
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Perform a check of all refrigerant side thermistors. . ; Perform a check of the refrigerant pressure sensor. . ; | Troubleshooting ; Check that all stop valves of the refrigerant circuit are open. . ; Check if the refrigerant circuit is clogged. . ; Check if the refrigerant circuit is correctly charged. . ; Check for the presence of non‑condensables and/or humidity in the refrigerant circuit. . ; Perform a check of the compressor. . ; Perform a check of the expansion valve. . ; Check for leaks in the refrigerant circuit. Look for oil traces on the unit(s). Check the brazing points on the field piping. Perform a pressure test, see "5.2 Refrigerant circuit " [ 4 256 ] .
Who to ask for: F-gas certified heat pump engineer (Daikin-trained installer / Stand By Me partner).
Certification: UK F-gas certification (Category I) for refrigerant circuit work; R-290 (Altherma 4) also needs flammable-refrigerant/hydrocarbon competence.
What you might notice
What owners and engineers report
11 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Keeps stopping / cutting out on the Altherma: 2 reports
- No hot water on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 68 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026