Pb on a Worcester Bosch Bosch Compress 2000 AWF: Alarm table: Antifreeze (this is not a protection, the alarm light does not flash), the remote control does not display Pb, but displays the antifreeze icon / Troubleshooting table: Antifreeze protection
Short answer
On a Worcester Bosch heat pump, Pb means: Alarm table: Antifreeze (this is not a protection, the alarm light does not flash), the remote control does not display Pb, but displays the antifreeze icon / Troubleshooting table: Antifreeze protection. 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: Alarm table: Antifreeze (this is not a protection, the alarm light does not flash), the remote control does not display Pb, but displays the antifreeze icon / Troubleshooting table: Antifreeze protection.
Part involved: antifreeze function.
When it shows: Antifreeze (this is not a protection, the alarm light does not flash), the remote control does not display Pb, but displays the antifreeze icon.
Likely causes:
- Antifreeze function active; unit automatically returns to normal operation.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Information only |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes.
Manual reset: Manual 13.2: switch the unit off and on again to reset the safety device; if reset fails contact dealer. Manual 11.4 warns repeated resets can cause irreversible damage.
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).
- Manual 13.2: one off/on restart may be tried; if the code returns, stop and call an engineer - repeated resets can damage the unit.
What the engineer will check
- None required unless persistent.
Who to ask for: heat pump installer / heating engineer.
Certification: None specific for hydraulic checks; MCS-certified installer recommended for system changes.
What you might notice
Sources
- Installation and operating instructions Compress 2000 AWF CS2000AWF 4-30 R-S/T ↗ · 6721859133 (2025/10) · page 128; 138 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026