Er66 on a Climaveneta / De Longhi Professional heat pump: Faulty domestic hot water sensor BT4
Short answer
On a Climaveneta / De Longhi Professional heat pump, Er66 means: Faulty domestic hot water sensor BT4. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
On the De'Longhi Professional / Climaveneta AWR MTD reverse-cycle air/water heat pump with DHW
The manual says: Faulty domestic hot water sensor BT4.
Likely causes:
- Sensor/wiring.
On the De'Longhi Professional / Climaveneta BWR MTD and MTD2 brine-to-water heat pump
The manual says: Faulty domestic hot water sensor BT4.
Likely causes:
- Sensor or wiring fault.
Is it safe?
On the De'Longhi Professional / Climaveneta AWR MTD reverse-cycle air/water heat pump with DHW
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
On the De'Longhi Professional / Climaveneta BWR MTD and MTD2 brine-to-water heat pump
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
What you can check
- Note the code/LED pattern and the model from the rating plate, then contact a heat pump/refrigeration engineer.
- Check the heat pump's breaker/isolator at the consumer unit is on.
What the engineer will check
On the De'Longhi Professional / Climaveneta AWR MTD reverse-cycle air/water heat pump with DHW
- Check connections; replace sensor.
Who to ask for: refrigeration / heat pump engineer.
Certification: Electrical competence for work inside the unit.
On the De'Longhi Professional / Climaveneta BWR MTD and MTD2 brine-to-water heat pump
- Check electrical connections; replace component.
Who to ask for: refrigeration / heat pump engineer.
Certification: Electrical competence for work inside the unit.
What you might notice
Sources
- Climaveneta AWR MTD / AWR MTD/B 0011-0061 Installation - Operating - Service Manual (GB 01/2009) ↗ · GB 01/2009 · page 48 · Reviewed 23 September 2026
- Climaveneta WWR MTD - MTD/S, BWR MTD - MTD/S Installation and User Manual (GB 03/2010) ↗ · GB 03/2010 · page 49 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026