LJ on a Mitsubishi Electric Ecodan: DHW operation error (type of external plate HEX)
Short answer
On a Mitsubishi Electric heat pump, LJ means: DHW operation error (type of external plate HEX). 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: DHW operation error (type of external plate HEX).
Part involved: DHW tank thermistor THW5/THW5B; sanitary-side pump; external plate heat exchanger.
When it shows: DHW heating via external plate heat exchanger not progressing.
Likely causes:
- 1 DHW tank thermistor (THW5 / THW5B) detached from holder; 2 Reduced flow in sanitary circuit; 3 Water circulation pump (primary/sanitary) fault.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| 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 for disconnection of DHW tank thermistor; 2 Check flow; 3 Check water circulation pump function.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
What you might notice
What owners and engineers report
17 owner and engineer reports mention this.
- No hot water on the Ecodan: 9 reports
- Hot water not hot enough on the Ecodan: 8 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