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Given that you have an LG Air Conditioner, that code signifies a loss of communication, or a lack of communication, between outdoor and indoor units.
It might also indicate a voltage or power supply (internal to the unit) problem. Is yours a Split/Multi? or a Multi? model.
You are also advised to check all the wiring (esp. between units) since the loss of serial signal between units might mean a break in wiring between indoor and outdoor units... and also to "remove external pumps", though I have no idea why, nor exactly what that last advice means. Perhaps it means disconnect them, then re-connect them a minute later.
And go to the outdoor unit, take off its cover, and check the green and the red LEDs. The green should be On (unless the outdoor unit has lost power - in which case the communication error is explained ), and the red LED should be flickering normally, if it is steady red=ON, then the communications are not working. If the red LED is completely OFF, then the fault is probably in the outdoor PCB, or connections to the outdoor PCB... in which case, after checking those connections, replace the outdoor PCB as needed.
Basically if there's much wrong with regulation, switching on or off, or efficiency of the LG Neo Plasma device(s), then you should have them serviced. Hopefully it's a minor problem if so.
If you need more information or help, let me know more about what's going on. And I will advise you further.