U4 on a Daikin heat pump: Code shown on the remote controller for the installation error(s): No power supplied to outdoor or indoor unit (including phase interruption) (Flex outdoor installation manual, test-operation table)
Short answer
On a Daikin heat pump, U4 means: Code shown on the remote controller for the installation error(s): No power supplied to outdoor or indoor unit (including phase interruption) (Flex outdoor installation manual, test-operation table). The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the Altherma Flex Type
The manual says: Code shown on the remote controller for the installation error(s): No power supplied to outdoor or indoor unit (including phase interruption) (Flex outdoor installation manual, test-operation table).
Part involved: inter-unit communication / PCB combination.
When it shows: Detected during test operation / normal operation after installation errors listed.
Likely causes:
- No power supplied to outdoor or indoor unit (including phase interruption).
On the Altherma HT (high temperature split)
The manual says: Malfunction of transmission between indoor and outdoor unit.
Part involved: inter-unit communication / PCB combination.
When it shows: When transmission is not carried out no rmally for a certain amount of time.
Likely causes:
- Indoor to outdoor transmission wiring F1, F2 disconnection, short circuit or wrong wiring; Outdoor unit power supply is OFF; Defect of outdoor unit PC board; Defect of indoor unit PC board (A1P) ESIE09-08C; Wrong wiring between indoor unit PC boards. [A4P (1 ø) or A5P (3ø)] and A1P.
On the Altherma LT (low temperature split)
The manual says: Signal transmission error (indoor outdoor unit).
Part involved: inter-unit communication / PCB combination.
When it shows: When the data sent from the outdoor unit cannot be received normally, or when the content of the data is abnormal | When the data sent from the outdoor unit cannot be received normally, or when the content of the data is abnormal. 3 3.
Likely causes:
- Faulty outdoor unit PCB; Faulty hydro-box PCB; Hydro-box-outdoor unit signal transmission error due to wiring error; Hydro-box-outdoor unit signal transmission error due to disturbed power supply waveform; Hydro-box-outdoor unit signal transmission error due to breaking of wire in the connection wires between the Hydro-box and outdoor units; Hydro-box -outdoor unit signal transmission error due to breaking of wire in the connection; wires between the Hydro-box and outdoor units.
Is it safe?
On the Altherma Flex Type
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: After correcting the abnormality press BS3 RETURN on the outdoor PCB to reset and repeat the test operation (installer only).
On the Altherma HT (high temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
On the Altherma LT (low temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
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
On the Altherma Flex Type
- Installer remedial action per manual: open stop valves; correct phase sequence; check power wiring; check piping/wiring consistency; correct refrigerant charge by recalculation; repeat test operation within -20~35 C.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check indoor to outdoor transmission wiring F1, F2 disconnection, short circuit or wrong wiring; check outdoor unit power supply is OFF; check defect of outdoor unit PC board; check defect of indoor unit PC board (A1P) ESIE09-08C; check wrong wiring between indoor unit PC boards. [A4P (1 ø) or A5P (3ø)] and A1P.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
On the Altherma LT (low temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check faulty outdoor unit PCB; check faulty hydro-box PCB; check hydro-box-outdoor unit signal transmission error due to wiring error; check hydro-box-outdoor unit signal transmission error due to disturbed power supply waveform; check hydro-box-outdoor unit signal transmission error due to breaking of wire in the connection wires between the Hydro-box and outdoor units; check hydro-box -outdoor unit signal transmission error due to breaking of wire in the connection.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Installation manual Daikin Altherma Flex EMRQ8-16AAY1 (4PWEN61262-1B) ↗ · 4PWEN61262-1B · page 32 (printed 29) · Reviewed 23 September 2026
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 214 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 191 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026