What it means
Daikin's guides list it as "OU: Shortage of refrigerant", where OU means the outdoor unit.
What to check, in order
- Read the help text
On the controller, open the main menu and choose Malfunctioning to see the code and a short description. The help button gives a longer one. Note the code for your installer.
- Use emergency heating if you need it
Daikin's user reference guide says the backup heater can take over the heating when the heat pump fails. If emergency mode is set to Manual, you confirm this on the Malfunctioning screen. Expect higher electricity bills while it runs.
When to call your installer
Call your installer. The refrigerant circuit is sealed and only an F-gas certified engineer should work on it.
A repair visit will usually check the refrigerant charge and the pipe joints for leaks, which needs a qualified refrigeration engineer.
- Note the model and serial number. They're on a label on the rating plate on the outdoor unit, and on the indoor unit or cylinder.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Daikin Altherma U0-00 error code mean?
The outdoor unit has detected a shortage of refrigerant in the sealed refrigerant circuit. Daikin's guides list it as "OU: Shortage of refrigerant", where OU means the outdoor unit.
Can I fix Daikin Altherma U0-00 myself?
Engineer only (engineer visit). Start with: read the help text; use emergency heating if you need it. Call your installer. The refrigerant circuit is sealed and only an F-gas certified engineer should work on it.
Is Daikin Altherma U0-00 worth repairing?
Get a quote first. Needs an F-gas engineer to find and fix the cause, so get a quote first.
Checked against
- Daikin Altherma 3 R W installer reference guide 4P496758-1B (ERGA-D, EHBH/EHBX-D), troubleshooting and error codes, p. 89 (ManualsLib copy)
- Daikin Altherma 3 R ECH2O installer reference guide, error codes overview, p. 276 (ManualsLib copy)
- Daikin Altherma low temperature split user reference guide 4P499574-1A (EHVH04/08S23DAV), PDF
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.