U7 on a Daikin heat pump: Malfunction of transmission outdoor unit
Short answer
On a Daikin heat pump, U7 means: Malfunction of transmission outdoor unit. 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 · 2 sources
What it means
On the Altherma HT (high temperature split)
The manual says: Malfunction of transmission 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:
- Improper connection of transmission wiring between outdoor unit and external control adaptor for outdoor unit; Improper cool/heat selection; Improper cool/heat unified address (outdoor unit, external control adaptor for outdoor unit); Defect of outdoor unit PC board (A1P); Defect of external control adaptor for outdoor unit.
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 PM1 can not be received successively for 9 sec. ■ The abnormality is determined if the above fault conditions occurs once. ■ Fault counter is reset when the data from the PM1 can be successfully received | When the data sent from the PM1 can not be received successively for 9 sec. The abnormality is determined if the above fault conditions occurs once. Fault counter is reset when the data from the PM1 can be successfully received. 3 3.
Likely causes:
- Defective outdoor unit PCB.
Is it safe?
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 HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check improper connection of transmission wiring between outdoor unit and external control adaptor for outdoor unit; check improper cool/heat selection; check improper cool/heat unified address (outdoor unit, external control adaptor for outdoor unit); check defect of outdoor unit PC board (A1P); check defect of external control adaptor for outdoor unit.
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 defective outdoor unit PCB.
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
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 218 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 193 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026