U8-04 on a Daikin Altherma: Unknown USB device

What it means

The manual says: Unknown USB device.

Part involved: user interface / room thermostat.

When it shows: Unknown USB device.

Likely causes:

• Connected USB/SDcard to update the user interface or upload e‑configuration data is NOT USB mass storage device. The USB's format MUST be FAT‑32.

Is it safe?

The heat pump keeps running.

Resetting itself: no.

Manual reset: Manual reset via the user interface.

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 any room thermostat/controller display is lit and its batteries (if battery-powered) are not flat; check thermostat and schedule settings.
• 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

• Remove the USB/SDcard from the user interface.

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

What owners and engineers report

13 owner and engineer reports mention this.

• No heating on the Altherma: 7 reports
• Controller / thermostat problem on the Altherma: 6 reports

Sources

• Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 75 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026