MRCOOL mini split error code PC 40
Communication error between outdoor main board and inverter board. Also shown as PC40.
- TypeFault
- SystemCommunication
- Can I fix it?Technician job
- Applies toindoor unit display (four-character codes)
What PC 40 means on a MRCOOL
The two boards inside the outdoor unit (main control and compressor driver) stopped talking to each other.
On this platform, PC 40 is a fault: something has failed or is disconnected and needs diagnosis.
Common causes
- A loose ribbon or plug between the boards.
- A failed driver board or main board.
- Water or corrosion on the board connectors.
How to fix it yourself
Do the reset and the visual checks, but the underlying repair needs a technician with a meter and the service manual.
- Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
- With power off, open the outdoor service panel and reseat the connectors between the boards.
- Look for water intrusion or corrosion.
Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.
When to call a technician
Board-level fault; a technician should test and replace the failed board.
Which MRCOOL units show PC 40
This page applies to MRCOOL DIY 2nd/3rd Gen, DIY 4th Gen, DIY Multi-Zone, Advantage, Olympus and Easy Pro units on the indoor unit display (four-character codes). MRCOOL's own documentation lists PC 40 as "Communication error between outdoor main board and inverter board", the same definition used across the Midea platform (four-character codes) it is built on. If your unit shows a code with a different shape (for example a four-character code, or a plain number), it is from a different table; use the lookup with your exact code.
Where MRCOOL shows this code
The indoor display shows the code in place of the temperature. On DIY 4th Gen units a four-character code alternates between its two halves (EL, then 0C); write down both. The MRCOOL app also reports the current fault.
EH and EL are indoor-side faults, EC is an outdoor-side fault, and PC is an outdoor protection: the unit stopped itself to prevent damage.
Other MRCOOL codes
Same part of the system as PC 40:
PC 40 on other brands
The same characters do not always mean the same thing. Check the brand on your unit.
- Pioneer PC 40 Communication error between outdoor main board and inverter board
Questions people ask
Can I reset a MRCOOL PC 40 code myself?
Yes. Turn the system off at the breaker for five minutes, then restart it. If PC 40 comes back, the fault is real and needs the checks above.
Is it safe to keep running a MRCOOL showing PC 40?
The unit will usually refuse to run, or run with reduced output, until the fault is cleared. Repeated resets do not fix anything and can mask a problem that is getting worse.
What will a technician check for PC 40?
Board-level fault; a technician should test and replace the failed board. Ask for the measurements, not just a parts quote.
Sources
- MRCOOL DIY 3rd Gen and 4th Gen installation manuals (error code tables)
- Midea residential split service manual
Updated September 2026. Codes and definitions vary by model year. If your manual disagrees with this page, your manual wins. Send a correction.