UA-09 on a Daikin Altherma 4 H: Indoor/outdoor combination abnormality

What it means

The manual says: Indoor/outdoor combination abnormality.

Part involved: inter-unit communication / PCB combination.

When it shows: Indoor-outdoor unit transmission error.

Is it safe?

The heat pump stops until the fault clears or is reset.

Resetting itself: yes.

Manual reset: Automatic.

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.
• 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

• See Daikin service manual for this generation (not public).

Who to ask for: Heat pump installer (Daikin-trained).

Certification: None specific; MCS-certified installer recommended.

What you might notice

• Controller / thermostat problem
• No heating

Sources

• Installer reference guide Daikin Altherma 4 H W EPSK/EPSKS + EPBX(U) (4P773390-1C) ↗ · 4P773390-1C · page 221 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026