What it means
Daikin's guides call it an "Indoor/outdoor unit communication problem". U5-00 is a different code, for a communication problem with the user interface.
What to check, in order
- Check the outdoor unit has power
Daikin UK lists the outdoor unit being switched off as one cause. Check that the switch next to the outdoor unit and the circuit breakers for the heat pump are on.
- 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
If the power is on and the code stays, call your installer. Daikin UK names a faulty interconnection cable or a faulty control card as the other causes. Wiring checks are a job for a qualified engineer.
Quick answers
What does the Daikin Altherma U4-00 error code mean?
The indoor unit can't communicate with the outdoor unit, so the heat pump can't run. Daikin's guides call it an "Indoor/outdoor unit communication problem". U5-00 is a different code, for a communication problem with the user interface.
Can I fix Daikin Altherma U4-00 myself?
Easy (about 5 minutes). Start with: check the outdoor unit has power; read the help text; use emergency heating if you need it. If the power is on and the code stays, call your installer. Daikin UK names a faulty interconnection cable or a faulty control card as the other causes. Wiring checks are a job for a qualified engineer.
Is Daikin Altherma U4-00 worth repairing?
Worth fixing. If the outdoor unit had simply been switched off, restoring power clears it. Otherwise an engineer needs to test the cable and the boards.
Checked against
- Daikin Altherma 3 R W installer reference guide 4P496758-1B, error codes, p. 90 (ManualsLib copy)
- Daikin Altherma 3 R ECH2O installer reference guide, error codes overview, p. 276 (ManualsLib copy)
- Daikin UK: Homeowner support
- Daikin UK: Daikin Altherma Low Temperature Split Heat Pump user guide (2015), PDF
- 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.