U5 on a Daikin Altherma HT: Malfunction of transmission between remote control and indoor unit / failure of remote control PC board or setting during control by remote control

What it means

The manual says: Malfunction of transmission between remote control and indoor unit / failure of remote control PC board or setting during control by remote control.

Part involved: user interface / room thermostat.

When it shows: Normal transmission does not continue for specified period.

Likely causes:

• Malfunction of indoor unit remote control transmission; Connection of two main remote contro ls (when using 2 remote controls); Defect of indoor unit PC board (A1P); Defect of remote control PC board; Malfunction of transmission caused by noise.

Is it safe?

The manual does not say whether the heat pump stops.

Resetting itself: unknown.

What you can check

• Read the full error code shown on the user interface 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.

What the engineer will check

• Follow the service-manual troubleshooting flow chart for this code: check malfunction of indoor unit remote control transmission; check connection of two main remote contro ls (when using 2 remote controls); check defect of indoor unit PC board (A1P); check defect of remote control PC board; check malfunction of transmission caused by noise.

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
• Controller display blank

Sources

• Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 217 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026