LB on a Mitsubishi Electric Ecodan: High pressure protection
Short answer
On a Mitsubishi Electric heat pump, LB means: High pressure 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: High pressure protection.
Part involved: Plate heat exchanger / heating circuit flow / outdoor unit.
When it shows: High refrigerant pressure detected at FTC.
Likely causes:
- 1 Flow rate of the heating circuit reduced; 2 Plate heat exchanger clogged; 3 Outdoor unit failure (refrigerant volume, valve, LEV coil, crushed pipe).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated as auto-resetting; code remains until reset.
Manual reset: Installation manuals: press F4 (RESET) then F3 (Yes) on the main remote controller (older controllers: hold button E for 3 s); 'To cancel error codes please switch system off'. Fix the cause first.
What you can check
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. Check the primary circuit pressure gauge (typically about 1-2 bar when cold on sealed systems) and look for visible leaks; do not top up or bleed unless your installer has shown you how and the filling loop is then closed.
What the engineer will check
- 1 Check water circuit; 2 Check plate heat exchanger; 3 Check refrigerant volume, stop valves, LEV coil and pipework of outdoor unit (refer to outdoor service manual).
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A) / R32 / R290 as applicable to the outdoor unit.
What you might notice
What owners and engineers report
15 owner and engineer reports mention this.
- No heating on the Ecodan: 9 reports
- Keeps stopping / cutting out on the Ecodan: 6 reports
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 62 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026