LD on a Mitsubishi Electric heat pump: Boiler temperature thermistor (THWB1, THWB2) failure
Short answer
On a Mitsubishi Electric heat pump, LD means: Boiler temperature thermistor (THWB1, THWB2) failure. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 5 sources
What it means
On the Ecodan
The manuals for this range word it in different ways:
- Boiler temperature thermistor (THWB1, THWB2) failure.
- Boiler temperature thermistor (THWB1) failure (FTC7: Thermistor (Boiler flow water temp.) (THWB1)).
Part involved: Boiler thermistors THWB1, THWB2; Boiler flow thermistor THWB1.
When it shows: THWB1/THWB2 open (-40C or below) or short (140C or above) THWB1 open (-40C or below) or short (140C or above).
Likely causes:
- 1 Connector/terminal detached or loose; 2 Thermistor fault; 3 FTC board failure.
On the Ecodan Hydrodan
The manual says: Boiler temperature thermistor (THWB1) failure (FTC7: Thermistor (Boiler flow water temp.) (THWB1)).
Part involved: Boiler flow thermistor THWB1.
When it shows: THWB1 open (-40C or below) or short (140C or above).
Likely causes:
- 1 Connector/terminal detached or loose; 2 Thermistor fault; 3 FTC board failure.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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.
What the engineer will check
- Check resistance across the thermistor; check connections; replace board if needed.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
What owners and engineers report
9 owner and engineer reports mention this.
- No heating on the Ecodan: 9 reports
Sources
- Ecodan EHSC-VM2B / EHPX-VM2B Service Manual ↗ · OCH532A · page 53 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan FTC5 - EHPT20X-MHCW, EHST20(D)(C)-MHCW Cylinder Service Manual (OCH570C) ↗ · OCH570C · page 64 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MHEDWR1 Service Manual (OCH714E) ↗ · OCH714E · page 56 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 60 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan Hydrodan EHWT17D-MHEDW Installation Manual RK79D203K04 ↗ · RK79D203K04 · page 48 (PDF p.49, Error Codes (FTC)) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026