Mitsubishi Electric U1 fault codewhat it means and how to fix it
High pressure: the outdoor unit’s 63H pressure switch has tripped, at 4.15 MPa on R410A and R32 Ecodans and 3.2 MPa on the R290 range. Mitsubishi say it is usually water flow.
Take care
Get an F-Gas certified heat pump engineer to look at your Mitsubishi Electric U1
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
Which Mitsubishi Electric appliance is showing U1?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
An Ecodan heat pump with radiators, underfloor heating or a hot water cylinder
This page is yours. U1 is high pressure, and Mitsubishi list water-flow causes before anything in the outdoor unit.
Check radiator valves, system pressure and the outdoor fan, reset once, then book your installer if it returns.
A wall-mounted air conditioner or a ceiling cassette
Same code and the same high pressure switch, but Mitsubishi’s published checks for an air conditioner are airflow and refrigerant, not a water circuit it does not have.
Use the Mitsubishi Electric U1 page in the air-conditioning section of this site.
What U1 means
On an Ecodan, U1 is high pressure. The outdoor unit service manuals define it as abnormal if the high pressure switch, which Mitsubishi label 63H, operates during compressor operation: at 4.15 MPa on the R410A PUHZ-W and PUHZ-HW units and the R32 PUZ-WM units, and at 3.2 MPa on the R290 PUZ-WZ range. It is the same fault on Mitsubishi’s air conditioners, the same switch and the same code, so this is not a case of one string meaning two things; what differs is the checks, because an air conditioner has no water circuit, and our air-conditioning Mitsubishi U1 page walks the other set. If your machine heats radiators or a cylinder, you are on the right page.
What makes the Ecodan U1 interesting is the order of the sixteen causes in the PUHZ service manual. The first five are all on the water side: decreased water flow, a clogged filter on the water pipe, dirt in the plate heat exchanger, a locked water pump, a malfunctioning water pump. A heat pump moves heat out of the refrigerant into the water, and if the water is not moving, the heat has nowhere to go, the refrigerant pressure climbs and the switch opens to protect the compressor. Only after those five does the list reach the outdoor unit itself: a locked or failed fan motor, short cycling of the outdoor air, a dirty outdoor heat exchanger, and an outside temperature thermistor reading low and slowing the fan. Then come the electrical possibilities, a disconnected 63H connector or a failed controller board, which Mitsubishi tell the engineer to test by powering off and seeing whether F5, the open-63H-connector code, appears on power-up.
Mitsubishi’s UK lookup says the same thing in one line for the Ecodan: usually caused by abnormal water flow on the primary circuit, check the flow rate, the pumps and zone valves, the water pressure and air in the primary circuit; and their engineer training files U1 under site-related faults as "high refrigerant pressure – flow related". So U1 is very often a protection working as designed against a circulation problem, and the compressor is usually fine. The outdoor unit is where the refrigerant lives. On R410A and R32 units that is a fluorinated gas and opening the circuit is F-Gas certified work by law; on the R290 range it is a highly flammable hydrocarbon that Mitsubishi restrict to trained installers. But the water-side causes are in the house, and some of them you can see.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Reduced water flow through the system | Mitsubishi’s first cause. Radiator valves all shut, a zone valve stuck closed or the pump on too low a setting leaves the plate heat exchanger with nowhere to put its heat. | Common |
| Clogged strainer or dirty plate heat exchanger | Causes two and three. Debris from the heating circuit collects in the filter and in the exchanger plates, and both restrict flow. | Common |
| Outdoor fan or airflow problem | Causes seven to eleven: a locked or failed fan motor, recirculated air, a dirty outdoor coil, or a thermistor telling the fan it is colder than it is. | Occasional |
| Pressure switch wiring or controller board | Causes twelve to fourteen in the PUHZ manual. Mitsubishi test these by powering off and watching for F5 at restart, which is the connector-open code for the same switch. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£600
Time on site
20 minutes
A strainer clean or a stuck valve freed is a short visit, typically £90 to £150. A fan motor or a pressure switch is a few hundred pounds fitted. The top of the range is a plate heat exchanger that has to be flushed or replaced.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Mitsubishi Electric fault codes
- Outdoor unit failureGet it checkedAny code starting U, F or A means the outdoor unit; Mitsubishi publish that rule themselves. It is the refrigerant side: F-Gas work on R410A and R32 Ecodans, trained-installer work on the R290 range.Related: refrigerant circuit
- L9Get it checkedLow flow on the primary circuit. Formally an installer job, but the causes are things you can often see — a dirty filter, a closed valve, air after radiator work.Related: same brand
- No code and nothing happeningGet it checkedMitsubishi resolve four in five support calls on the phone, and publish the four common issues behind them. Three take two minutes and none of them is a code.Related: same brand
- EFGet it checkedA code Mitsubishi define as "non defined": noise on the wiring between the controller, the FTC and the outdoor unit. Their published remedy is to power the Ecodan down for thirty seconds.Related: same brand
- L3Get it checkedCirculation water temperature overheat protection. The flow rate has probably dropped, and Mitsubishi list three things to check: a water leak, a blocked strainer and the pump.Related: same brand
- J1Get it checkedThe wireless receiver cannot hear the wireless room controller. Mitsubishi’s first check is the controller’s batteries, and J1 to J8 all mean the same thing.Related: same brand
Own a Mitsubishi Electric heat pump? Save it and its codes come first next time.
Related guides
Common questions about Mitsubishi Electric U1
Is this the same U1 as on a Mitsubishi air conditioner?
The same fault, with different checks. On both it is the outdoor unit’s high pressure switch, 63H, operating while the compressor runs; Mitsubishi’s UK lookup returns one row headed Ecodan high pressure fault whose checks are all water circuit, and one for the air conditioners. If your Mitsubishi blows air rather than heating water, the page for you is in our air-conditioning section, because none of the water checks here apply.
Why would a water problem show as a refrigerant pressure fault?
Because the plate heat exchanger is where refrigerant hands its heat to water. Stop the water and the refrigerant cannot cool, so its pressure rises until the switch opens. That is why Mitsubishi put decreased water flow, a clogged filter and a dirty exchanger at the head of the cause list, and why their lookup says U1 is usually caused by abnormal water flow.
Does the engineer need F-Gas certification?
On a PUHZ-W, PUHZ-HW, PUZ-WM or PUZ-HWM unit, yes, to open the outdoor unit and touch the pressure switch, the expansion valve or the charge: those contain R410A or R32, both fluorinated gases. On the PUZ-WZ Ecodan R290 range the refrigerant is not an F-gas, so the certificate is not the legal test, but it is highly flammable and Mitsubishi restrict the unit to trained installers. To clean a strainer or free a water pump on the indoor side, neither applies. A good installer will start indoors, because that is where Mitsubishi’s list starts.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.