L4 on a Worcester Bosch Bosch Compress 2000 AWF: Alarm table: MC/synchronisation/closed circuit error / Troubleshooting table: MCE protection
Short answer
On a Worcester Bosch heat pump, L4 means: Alarm table: MC/synchronisation/closed circuit error / Troubleshooting table: MCE protection. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Alarm table: MC/synchronisation/closed circuit error / Troubleshooting table: MCE protection.
Part involved: compressor drive.
When it shows: MC/synchronisation/closed circuit error.
Likely causes:
- MCE protection (see L0 check list).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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.
- 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
- Check compressor wiring and inverter.
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
What owners and engineers report
1 owner and engineer report mentions this.
- Keeps stopping / cutting out on the Bosch Compress 2000 AWF (R32 all-in-one monobloc): 1 report
Sources
- Installation and operating instructions Compress 2000 AWF CS2000AWF 4-30 R-S/T ↗ · 6721859133 (2025/10) · page 129; 139 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026