L9 on a Daikin Altherma HT: Inverter start up error
Short answer
On a Daikin heat pump, L9 means: Inverter start up error. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Inverter start up error.
Part involved: compressor / inverter drive.
When it shows: When overload in the compressor is detected during startup.
Likely causes:
- Defect of compressor; Pressure differential start; Defect of outdoor unit PC board (A2P); Defect of outdoor unit PC board [A4P (1 ø) or A5P (3ø)].
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
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 defect of compressor; check pressure differential start; check defect of outdoor unit PC board (A2P); check defect of outdoor unit PC board [A4P (1 ø) or A5P (3ø)].
Who to ask for: F-gas certified heat pump engineer with electrical competence.
Certification: UK F-gas certification if the refrigerant circuit is opened; electrical competence (BS 7671) for inverter/PCB work.
What you might notice
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 200 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026