MRCOOL code P4 / PC04

MRCOOL P4 and PC-04 error code: inverter compressor drive fault

MRCOOL P4, shown as PC-04 on 4th generation DIY units, is an inverter compressor drive error. What generation naming to expect, the safe power-off step, and when it needs support.

Source verifiedBy HVAC Bench Editorial DeskLast reviewed
Direct answer

What this means

MRCOOL uses this code for an inverter compressor drive fault, shown as P4 on generations before the fourth and as PC-04 on fourth generation DIY equipment. The drive electronics that run the compressor have reported an abnormal condition and stopped it. The cause differs between a newly installed system and one that has been running.

Equipment and model context

MRCOOL DIY mini-split generations that report an inverter compressor drive fault

  • DIY 3rd Gen
  • DIY 4th Gen
  • DIY 5th Gen
  • Advantage series

This meaning follows MRCOOL DIY support literature for the generations that display P4 or PC-04. The generation matters more than the digits, because MRCOOL renumbers codes between series, so identify the DIY generation from the model number before using this page.

What you may notice

  • The indoor display shows P4 or PC-04 and the outdoor unit stops, either soon after start-up or after a period of running.
  • On a new installation the system may never have reached full output, while on an older one it ran normally until the code appeared.

Likely causes

  • On a newly installed system, a wiring error between indoor and outdoor units, a low supply voltage, or a stop valve left closed can push the drive outside its limits at start-up.
  • On a running system, a failing run capacitor or contactor, restricted outdoor airflow raising pressures, or a fault in the inverter module or compressor can trip the drive.

Diagnostic paths

P4 or PC-04 on a system installed in the last few weeks

The DIY system is new, it has not run a full cooling or heating cycle correctly, and the code appears early, sometimes right after the compressor tries to start.

Recheck the indoor to outdoor wiring against the manual, confirm the supply voltage at the outdoor unit is within range, and confirm both stop valves are fully open. A fresh-install drive fault points at wiring, supply, or a closed valve.

P4 or PC-04 on a system that ran normally for a season

The unit cooled and heated without trouble, then began stopping with the code, sometimes worse in hot weather or when the compressor ramps up.

Isolate power at the breaker for fifteen minutes as MRCOOL directs, then restart once. If the code returns, arrange a technician to check the capacitor, contactor, outdoor airflow, and the inverter module and compressor current.

What the compressor drive protects against

The inverter drive converts the mains supply into a variable output that runs the compressor. It watches its own current, voltage, and temperature, and P4 or PC-04 is it stopping on one of those limits.

  1. Mains supplyFeeds the outdoor unit at the rated voltage
  2. Inverter driveConverts supply into a variable feed for the compressor
  3. CompressorThe load the drive is controlling
  4. Drive self-monitoringWatches current, bus voltage, and module temperature
  5. P4 or PC-04A limit is exceeded, the drive stops the compressor
Reading a P4 or PC-04 by system history
System ageMore likely areaFirst step
New DIY installWiring, supply voltage, or a closed stop valveRecheck wiring, voltage, and both valves
Ran a season, then started trippingCapacitor, contactor, outdoor airflow, or the driveFifteen-minute power-off, one restart, then service
Only in hot weather at full loadHigh pressures from restricted outdoor airflowClear the outdoor coil and space around the unit

The generation tells you which code you have

MRCOOL changes its code numbering between DIY generations, so the same underlying fault shows as P4 on earlier series and as PC-04 on the fourth generation. Identifying the generation from the model number is the first move, because a support article, a manual, and a parts list are all generation specific. A fifth generation unit uses a different refrigerant and a different board again, so a code table borrowed from the wrong generation can send a diagnosis the wrong way.

Whatever the label, the code is the compressor drive stopping on one of its own protection limits. The drive turns the fixed mains supply into a variable feed for the compressor and watches its current, its internal bus voltage, and the temperature of its power module. When any of those goes outside the safe window, it shuts the compressor down rather than risk damage.

Start with the safe power-off, then decide

MRCOOL's first instruction for this code is to isolate power at the breaker and leave it off for fifteen minutes before touching anything, so the drive fully de-energises and its capacitors discharge. After that, one restart tells you whether the fault was a single event or a standing problem. If the system runs normally afterward, note the date and watch it. If P4 or PC-04 returns, the drive is reporting a real condition.

From there the branches split by history. A new install points at the wiring between the units, the supply voltage, or a stop valve left closed, all of which the installer can recheck. A system that has run for a season points at the run capacitor, the contactor, outdoor airflow raising the pressures, or the inverter module and compressor themselves, which need current readings on a meter. Those measurements are technician work.

Service handoff

What to give the technician

Give the technician the DIY generation and model number, whether the code is P4 or PC-04, whether the system is new or has run a season, whether it is worse in hot weather, and whether the fifteen-minute power-off changed anything.

Homeowner safe

Safe homeowner checks

  1. Identify the DIY generation from the model number so the correct code table and parts list are used.
  2. Isolate power at the breaker for fifteen minutes, then restart once, and note whether P4 or PC-04 returns.

Is a reset appropriate?

One restart after the fifteen-minute power-off is the reset MRCOOL describes. Repeating it while P4 or PC-04 returns stresses the drive and the compressor and removes useful timing information.

Stop point

When to call a technician

  • Testing the run capacitor, contactor, inverter module, and compressor current, and checking supply voltage under load, require a qualified technician working at energised terminals.

Questions people ask about this

Is MRCOOL P4 the same as PC-04?

They are the same fault at different generations. Earlier DIY series show P4 and fourth generation units show PC-04. Identify the generation from the model number so the right manual is used.

What is the fifteen-minute power-off for?

It lets the inverter drive fully de-energise and its capacitors discharge before any check, and it gives the drive a clean restart. MRCOOL lists it as the first step for this code.

Why does P4 happen on a brand-new MRCOOL install?

On a fresh install the code points at the wiring between the indoor and outdoor units, a supply voltage below range, or a stop valve left closed. The installer should recheck all three.

Evidence record

How this page was checked

official manufacturer support article · checked 2026-09-09

Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: MRCOOL 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
Scope of the definition
MRCOOL DIY mini-split generations that report an inverter compressor drive fault
Last checked
2026-09-09