5378 on a Worcester Bosch Bosch Compress 5800i AW: Info: outdoor unit defrost fault
Short answer
On a Worcester Bosch heat pump, 5378 means: Info: outdoor unit defrost fault. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Info: outdoor unit defrost fault.
Part involved: defrost, sensor TL2.
When it shows: Info: outdoor unit defrost fault.
Likely causes:
- Heating system temperature/flow temperature too low; evaporator air flow too low; sensor TL2 defective.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: Clears when cause removed.
Manual reset: Reset/acknowledge via UI 800 fault menu after cause removed.
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
- Check the outdoor unit is not blocked by leaves, snow, ice or other debris (do not open the casing).
- Make sure radiator/UFH thermostats are not all closed.
What the engineer will check
- Open more thermostats; clean evaporator; check TL2 against sensor tables and replace.
Who to ask for: F-gas refrigeration engineer / heat pump installer.
Certification: F-gas certification (Category I) required for refrigerant circuit work; qualified electrician for electrical work.
What you might notice
Sources
- Installationsanleitung Bedienfeld UI 800 (Luft-Wasser-Wärmepumpe) 6721833375 ↗ · 6721833375 (2023/09) DE, AT, CH, LU · page 21 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026