A01-5378 on a Worcester Bosch heat pump: Outdoor unit defrost failure
Short answer
On a Worcester Bosch heat pump, A01-5378 means: Outdoor unit defrost failure. 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: Outdoor unit defrost failure.
Part involved: outdoor unit defrost, sensor TL2.
When it shows: Outdoor unit defrost failure.
Likely causes:
- Heating system temperature too low (open more thermostats); defective sensor TL2.
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 is removed.
Manual reset: User guides (7001i/7400i AWE, 3400i AWS, section 3.5): user may confirm the fault by pressing the selector; if it persists call the contractor with fault code, sub-code and user-interface ID no.
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
- Make sure radiator/UFH thermostats are not all closed.
- Check the outdoor unit is not blocked by leaves, snow, ice or other debris (do not open the casing).
- Confirm/acknowledge the fault once at the controller (press selector) as the user guide allows; if it returns, call the installer.
What the engineer will check
- Check TL2 against sensor table and replace if deviating.
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
- Installation instructions User interface HPC 410 ↗ · 6721832682 (2021/02) · page 19 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026