C1 on a Daikin Altherma HT: Failure of transmission (between indoor unit PC board and fan PC board)
Short answer
On a Daikin heat pump, C1 means: Failure of transmission (between indoor unit PC board and fan PC board). 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: Failure of transmission (between indoor unit PC board and fan PC board).
Part involved: inter-unit communication / PCB combination.
When it shows: When normal transmission is not conducted for certain duration.
Likely causes:
- Connection defect of the connecter between indoor PC board (A1P) and PC board for control (A3P) (communication problem); Malfunction of indoor PC board (A1P); Malfunction of PC board for control (A3P); External factor, such as instantaneous blackout. ESIE09-08C.
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.
What the engineer will check
- Follow the service-manual troubleshooting flow chart for this code: check connection defect of the connecter between indoor PC board (A1P) and PC board for control (A3P) (communication problem); check malfunction of indoor PC board (A1P); check malfunction of PC board for control (A3P); check external factor, such as instantaneous blackout. ESIE09-08C.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
What you might notice
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 152 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026