L1 on a Daikin Altherma HT: Malfunction of PC board

What it means

The manual says: Malfunction of PC board.

Part involved: PCB / EEPROM.

When it shows:  In case of overcurrent (OCP) during waveform output  When the current sensor malfuncti ons during synchronized operation  In case of IGBT malfunction.

Likely causes:

• Faulty outdoor PC board (A1P)  Over current  Current sensor failure  Failure of IGBT or drive circuit; Faulty indoor PC board [A4P (1 ø) or A5P (3ø)]  Over current  Current sensor failure  Failure of IGBT or drive circuit.

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 the heat pump isolator switch(es) and the consumer-unit breaker/RCD for the heat pump are ON. If a breaker or RCD has tripped, do not keep resetting it - leave it and call an engineer.

What the engineer will check

• Follow the service-manual troubleshooting flow chart for this code: check faulty outdoor PC board (A1P)  Over current  Current sensor failure  Failure of IGBT or drive circuit; check faulty indoor PC board [A4P (1 ø) or A5P (3ø)]  Over current  Current sensor failure  Failure of IGBT or drive circuit.

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

Certification: Electrical competence to BS 7671 for PCB replacement.

What you might notice

• Heat pump will not start
• No heating

Sources

• Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 192 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026