UL on a Mitsubishi Electric heat pump: Low pressure (abnormal low pressure detected after compressor starts heating operation)
Short answer
On a Mitsubishi Electric heat pump, UL means: Low pressure (abnormal low pressure detected after compressor starts heating operation). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 8 sources
What it means
On the Ecodan Hydrodan
The manual says: Low pressure (abnormal low pressure detected after compressor starts heating operation).
Part involved: Refrigerant circuit / linear expansion valve.
When it shows: TH33 - TH4 > 20C and TH33 > 80C during compressor operation.
Likely causes:
- 1 Defective linear expansion valve; 2 Defective C.B.
On the Ecodan Puhz Sw Split
The manual says: Low pressure (63L operated) (SW100/120 only): 63L operates below -0.03 MPa during compressor operation.
Part involved: Low pressure switch 63L.
When it shows: 63L operated (under -0.03 MPa) during compressor operation.
Likely causes:
- 1 Outdoor stop valve closed during operation; 2 Loose 63L connector on outdoor controller board; 3 Loose 63L connection; 4 Defective outdoor controller board; 5 Leakage or shortage of refrigerant; 6 Malfunction of linear expansion valve.
On the Ecodan Puhz W Monobloc
The manual says: Low pressure (63L operated): 63L operates below -0.03 MPa during compressor operation.
Part involved: Low pressure switch 63L.
When it shows: 63L operated (under -0.03 MPa) during compressor operation.
Likely causes:
- 1 Outdoor stop valve closed during operation; 2 Loose 63L connector on outdoor controller board; 3 Loose 63L connection; 4 Defective outdoor controller board; 5 Leakage or shortage of refrigerant; 6 Malfunction of linear expansion valve.
On the Ecodan R290 Puz Wz
The manual says: Low pressure (abnormal low pressure detected after compressor starts heating operation).
Part involved: Refrigerant circuit / linear expansion valve.
When it shows: After 3 minutes of heating operation: TH33 - TH4 >= 20C and TH33 >= 80C.
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Clogging with foreign objects in refrigerant circuit (water ingress freezing).
On the Ecodan R32 Puz Wm
The manual says: Low pressure (abnormal low pressure detected after compressor starts heating operation).
Part involved: Refrigerant circuit / linear expansion valve.
When it shows: After 3 min of heating: TH33 - TH4 >= 20C and TH33 >= 80C.
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Clogging with foreign objects in refrigerant circuit (water ingress freezing); 4 Disconnection or contact failure of connector (63L) on outdoor controller board.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Low pressure (63L operated) (W112 only): 63L operates below -0.03 MPa during compressor operation.
Part involved: Low pressure switch 63L.
When it shows: 63L operated (under -0.03 MPa) during compressor operation.
Likely causes:
- 1 Outdoor stop valve closed during operation; 2 Loose 63L connector on outdoor controller board; 3 Loose 63L connection; 4 Defective outdoor controller board; 5 Leakage or shortage of refrigerant; 6 Malfunction of linear expansion valve.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Low pressure (63L operated): 63L operates below -0.03 MPa during compressor operation.
Part involved: Low pressure switch 63L.
When it shows: 63L operated (under -0.03 MPa) during compressor operation.
Likely causes:
- 1 Outdoor stop valve closed during operation; 2 Loose 63L connector on outdoor controller board; 3 Loose 63L connection; 4 Defective outdoor controller board; 5 Leakage or shortage of refrigerant; 6 Malfunction of linear expansion valve.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Low pressure (63L operated): 63L operates below -0.03 MPa during compressor operation.
Part involved: Low pressure switch 63L.
When it shows: 63L operated (under -0.03 MPa) during compressor operation.
Likely causes:
- 1 Outdoor stop valve closed during operation; 2 Loose 63L connector on outdoor controller board; 3 Loose 63L connection; 4 Defective outdoor controller board; 5 Leakage or shortage of refrigerant; 6 Malfunction of linear expansion valve.
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-recovering in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
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
On the Ecodan Hydrodan
- 1 Check LEV; 2 Replace C.B.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable).
On the Ecodan Puhz Sw Split
- 1 Check stop valve; 2-4 Power off/on and check if F3 is displayed, follow F3 action; 5 Correct refrigerant amount; 6 Check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Puhz W Monobloc
- 1 Check stop valve; 2-4 Power off/on and check if F3 is displayed, follow F3 action; 5 Correct refrigerant amount; 6 Check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan R290 Puz Wz
- 1 Check intake superheat, check refrigerant leakage and charge; 2 Check linear expansion valve; 3 Recover refrigerant, evacuate circuit >1 hour to remove water.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification plus competence for flammable A3 refrigerant (R290).
On the Ecodan R32 Puz Wm
- 1 Check intake superheat, refrigerant leakage and charge; 2 Check LEV; 3 Recover refrigerant and evacuate >1 hour; 4 Check short wiring connected to connector 63L.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable).
On the Ecodan Ultra Quiet Puhz W Vaa
- 1 Check stop valve; 2-4 Power off/on and check if F3 is displayed, follow F3 action; 5 Correct refrigerant amount; 6 Check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1 Check stop valve; 2-4 Power off/on and check if F3 is displayed, follow F3 action; 5 Correct refrigerant amount; 6 Check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Zubadan Puhz Shw Split
- 1 Check stop valve; 2-4 Power off/on and check if F3 is displayed, follow F3 action; 5 Correct refrigerant amount; 6 Check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- Insufficient heating (house not warm enough) on the Ecodan Zubadan Puhz Hw Monobloc: 1 report
Sources
- Ecodan Hydrodan EHWT17D-MHEDW Installation Manual RK79D203K04 ↗ · RK79D203K04 · page 51 (PDF p.52) · Reviewed 23 September 2026
- Ecodan PUHZ-SW(75-100-120)(V-Y)HA-BS Service Manual (OCH755) ↗ · OCH755 · page OCH755 p.27 · Reviewed 23 September 2026
- Ecodan PUHZ-W112VHA-BS Service Manual (OCH562D) ↗ · OCH562D · page OCH562D p.22; OCH439N p.59 · Reviewed 23 September 2026
- PUZ-WZ(85-100-120)(V-Y)AA(-BS) Service_Manual (OCH880) ↗ · OCH880 · page OCH880 p.42; OCH816 p.38 · Reviewed 23 September 2026
- Ecodan PUZ-WM(50-60-85-112)V(H-A)A Service Manual (OCH727A) ↗ · OCH727A · page 29 · Reviewed 23 September 2026
- PUHZ-W85-112VAA(-BS) Service Manual (OCH681) ↗ · OCH681 · page 19 · Reviewed 23 September 2026
- Ecodan PUHZ-HW140VHA2-BS / PUHZ-HW140YHA2-BS Service Manual (OCH439N) ↗ · OCH439N · page 59 · Reviewed 23 September 2026
- PUHZ-SHW112-140V/YHA Service Manual (OCH526) ↗ · OCH526 · page 29 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026