MRCOOL air conditioner P4 error code: inverter compressor drive error
Also shown as PC 04
- Urgency
- High
- Who can fix it
- Technician job
- Time
- About 30 min
- Repair cost
- $300–$2500
What the P4 code means
P4 on a MRCOOL mini-split is an inverter compressor drive error. Gen 4 and Gen 5 units show PC 04. On a new install, MRCOOL says to check first that the service valves are open and every line-set connection is tight; on older systems, low charge or a board or compressor fault is likely.
The inverter board drives the compressor at variable speed. If the compressor draws an odd current or does not respond as expected, the board stops it and shows P4.
On Gen 1 to 3 multi-zone units, P4 on the outdoor display means compressor discharge temperature protection instead. The checks are similar, but tell the technician which display showed the code.
Common causes
Listed from most to least common.
- Outdoor service valves not fully opened after installation
- Line-set connection loose, so refrigerant is escaping
- Low refrigerant charge on an older system
- On multi-zone systems, communication wires crossed between zones
- Loose plug or burn marks on the outdoor board
- Compressor or inverter board fault
How to fix the P4 code
Unplug the air conditioner or switch it off at the breaker before removing any panel or filter.
Step 1: Turn off the power
Switch off the breaker and disconnect, and leave them off for 15 minutes as MRCOOL advises.
Step 2: Check the valves and connections
On a new install, make sure both service valves are fully open, including the two king valves on multi-zone units. Check that every line-set connection is tight with no threads showing.
Step 3: Check multi-zone wiring
Each air handler's wires must go to the terminals that match its pipe ports. The unit on the A ports goes to 1A, 2A and 3A, and so on.
Step 4: Look for oily residue
On a system that had been running, check the line set, insulation and fittings for oily spots that point to a leak.
Step 5: Restore power and test
Turn the power back on and run the system. If P4 returns, book a technician.
You'll need: Allen key (hex) wrench set, two adjustable wrenches, flashlight.
How to reset the MRCOOL air conditioner
- Turn the mini-split off with the remote.
- Switch off the breaker for the system and the outdoor disconnect switch. MRCOOL suggests leaving the power off for 15 minutes so the boards fully reset.
- Restore power and turn the unit back on. The compressor normally waits about 3 minutes before starting, so give it time before judging.
- If the same code comes back straight away, the fault is still there. Work through the fix steps above, or call a technician.
When to call a technician
If P4 returns with the valves open and connections tight, a technician needs to check the refrigerant charge and test the compressor windings and outdoor board. Do not open the outdoor electrical box. Refrigerant work requires EPA certification.
A typical repair for this code costs $300–$2500 in the US, including parts and labor. Prices vary by area and model.
Models this applies to
- DIY Series ductless mini-splits (Gen 1 to 5), single-zone and multi-zone
- Advantage Series mini-splits
- Olympus Series mini-splits
- EZ-PRO mini-splits
Codes can differ slightly between models. Your owner's manual is the final word and is available from MRCOOL support.
Frequently asked questions
›Does P4 mean my compressor is dead?
Not always. On a new install, a closed valve or loose connection causes it more often. On older systems the board, the charge or the compressor can all be at fault.
›What is the CE reset on a multi-zone MRCOOL?
It is a reset done with a button by the outdoor unit's LED display, with the power on. MRCOOL suggests it on multi-zone systems when wiring is hard to trace. Because the outdoor unit is live, have a technician do it.
›Will warranty cover a refrigerant top-off for P4?
MRCOOL's warranty does not cover labor or maintenance. Keep the service invoice, as MRCOOL may ask for the charge levels.