Mitsubishi U6: the inverter cut the compressor current before anything melted
Mitsubishi U6 reports a compressor overcurrent or power module detection at the outdoor unit. Learn what the inverter measured and why it names no single part.
What this means
U6 is raised when the outdoor inverter detects compressor current or power module conditions outside the range it will drive. It is an electrical protection acting in milliseconds, which is why the code arrives as an abrupt stop rather than a gradual loss of output. The detection names the current, not the cause of the current.
Equipment and model context
Mitsubishi Electric inverter outdoor units publishing the U-series fault table, including M-Series single-zone and MXZ multi-zone equipment
- MUZ-GL series
- MUZ-FS series
- MXZ-C series
- MXZ-SM series
The entry described here belongs to Mitsubishi Electric outdoor units publishing the U-series table. Multi-zone outdoor units serving several indoor heads report the code centrally, so the address shown does not identify a room. Confirm the outdoor model from its plate before applying this reading.
What you may notice
- The outdoor unit starts, runs briefly or not at all, and stops abruptly, with U6 shown on the indoor display or a wired controller.
- The stop repeats at a similar point in the start sequence each time rather than at random intervals during steady running.
- House lights dim or another appliance stutters at the moment the outdoor unit attempts to start.
Likely causes
- A compressor that is hard to turn, for instance after liquid refrigerant has collected in it during a long off period, draws more current at start than the drive will supply.
- Supply voltage that sags at the moment of start makes the drive pull more current for the same mechanical work.
- The power module or its associated sensing circuit can itself detect and report conditions that have no mechanical cause at all.
Diagnostic paths
The stop happens in the first seconds of every start
Each attempt fails at roughly the same moment after the outdoor fan or compressor begins, and the timing is consistent over several tries.
Stop attempting starts and record how many seconds elapse before the stop. A repeatable start-window failure is a different investigation from an intermittent trip during steady running.
Lights flicker or other equipment reacts when it starts
Lighting dims briefly or another motor appliance hesitates in the same instant that the outdoor unit tries to start.
Note which circuits react and report it. Supply behaviour at the moment of start belongs to the electrical installation as much as to the heat pump, and both need looking at.
It follows a long shutdown or a hard freeze
The system had been switched off for weeks, or sat through severe cold without running, and the code appeared on the first attempt to use it again.
Leave the power on but the operation off so any crankcase heating arrangement can work, and report the length of the shutdown to service.
Why does a current limit exist at all? The drive converts supply into a controlled waveform, the compressor converts that into shaft work, and a current above the design band means one of those conversions is meeting resistance the drive was not built to push through.
- SupplyVoltage available at the outdoor unit
- Inverter driveBuilds the waveform the motor receives
- CompressorConverts that into refrigerant flow
- Current limitStops before semiconductors or windings suffer
| Timing | What that narrows | Owner action |
|---|---|---|
| Within seconds of every start | Starting torque or drive detection | Stop retrying and record the interval |
| After minutes of running | Load, airflow, or supply under duty | Record outdoor conditions at the stop |
| First use after a long layoff | Migration into the compressor | Leave power on and operation off |
| Alongside household flicker | Supply capacity at start | Report the affected circuits |
An electrical protection has to act faster than a diagnosis
Thermal protections have time to think. A discharge temperature limit watches a value climb over seconds or minutes and can be reasonably confident about what it means. A current protection has no such luxury: the semiconductors it defends can be destroyed in a fraction of a cycle, so the drive shuts down on the measurement alone and reports afterwards. That is the reason U6 names a detection rather than a component.
For the reader, the consequence is that the code alone narrows almost nothing. Mechanical resistance in the compressor, a supply problem outside the equipment, and a fault in the drive's own sensing all present as the same number on the display. The information that separates them comes from timing, from the supply, and from measurements taken with the panel open.
Why repeated restarts make this one worse
With a drain fault or a sensor fault, an extra restart costs nothing but time. With a current protection it is different. Every attempt subjects the power stage to the same event that triggered the trip, and if the underlying condition is mechanical, each attempt also asks a compressor that is already struggling to break away under full drive. A fault that was recoverable can become a replacement through persistence alone.
The disciplined response is to attempt one restart, observe carefully, and then leave the system alone. If the equipment has been off for a long period, leaving power connected while keeping operation off lets any crankcase heating arrangement do its job before the next attempt. The crankcase heater guide explains why that waiting period exists, and the outdoor unit not running reference covers the observations worth making while the system is stopped.
What to give the technician
Tell the technician the outdoor model and serial number, the U6 code, how many seconds the unit ran before each stop, whether household lighting reacted at start, and how long the system had been idle beforehand.
Safe homeowner checks
- Attempt at most one restart from the controller, then stop and note how long the outdoor unit ran before the code returned.
- Check whether other appliances on the property are behaving normally and whether any protective device in the consumer unit or panel has operated.
- Leave the isolator on and the operation off for several hours before any further attempt if the system has been unused for a long period.
Is a reset appropriate?
One controlled restart is reasonable to establish whether the stop repeats. Beyond that, each attempt repeats the electrical event the protection was defending against, so leave the system off until it is measured.
When to call a technician
- Measuring compressor winding resistance, drive output, and supply voltage under load requires opening an energised outdoor enclosure and is qualified work.
- A burning smell, visible scorching, or a protective device that trips when the unit starts is a stop condition. Isolate at the dedicated switch and arrange service without further attempts.
Questions people ask about this
Is U6 the same as a tripped breaker?
No. A breaker protects the building wiring and opens the supply. U6 is the drive protecting its own power stage and the compressor, and the supply stays live while the unit reports the stop.
Why does U6 appear after the system sat unused all summer?
Refrigerant can migrate into the compressor during a long idle period, which makes the first start much harder than a normal one. Leaving power connected before use lets any crankcase heating arrangement drive that off.
Can bad house wiring cause this code?
Supply voltage that sags under starting load makes the drive draw more current for the same work, which can reach the limit. That is why the electrical installation is examined alongside the heat pump rather than after it.
How many times can I safely retry?
One attempt establishes whether the fault repeats. Further attempts put the same stress on the power stage and on a compressor that may already be struggling, which can convert a diagnosable fault into a replacement.
Evidence recordHow this page was checked
official manufacturer support article, service manual · checked 2026-09-15
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Mitsubishi Electric technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article, service manual
- Scope of the definition
- Mitsubishi Electric inverter outdoor units publishing the U-series fault table, including M-Series single-zone and MXZ multi-zone equipment
- Last checked
- 2026-09-15