Only one unit works when only one unit is turned on. What is the problem?

Yeah, I'm not following what you mean, either.

You do know you have to have the LGs both set to the same setting (they can't be on auto because if the house isn't obviously cold or obviously warm, one might try to "cool" while the other "heats" and that makes an error...)

Apologies for the confusion. The split units are placed in two separate bedrooms.

Originally, we would select the one in the master bedroom, or the one in the guest room to operate either at the same time or each independently. When operating simultaneously, each unit kept its respective selected temperature.

Now, when we turn the master bedroom's unit on, it will not cool (the fan runs) unless the guest room's unit is selected on as well. In other words, we have to run both inside units to make the master bedroom's side cool. I hope this helps and again, thanks for your response(s)!

Is this a heat/cool unit or cool only? It sounds like the one the the guest room might be making the system "think" that it is in the heating mode when it is turned off. That would not allow the MBr unit to cool until the guest room unit is satisfied. The controller is only getting the correct handshake when the guest room unit is on and it is supposed to be getting it when it is off as well.

Too bad I don't know how to test my hypothesis.

This is a cool only unit. Thank you.

Thank you for your reply. I truly appreciate it!

Our LG dual has a "master" and a "slave" unit. Are you sure it worked the way you thought initially?

(Our LR unit is our master, and our FR unit is the slave. The LR unit calls the shots.)

I am on the road right now but when I return home, I'll check. From the great replies, it sounds as if the wiring needs to be corrected. While I'm there, what is a good UV proof wire for the 36v type circuit?

Thank you again!

I actually had something like this happen on a dual unit few years ago. It ended up that the control wires were crossed in the condensing unit. Swapped the wires and each zone ran individually.

Thanks. I'll have to follow that approach in solving this.

Good. To be more clear, you will have an A wire going to an A unit and a B to a B. What I think you have is a A-B and B-A.

Got it. Thank you!

Sry Old Tread, However I installed a Dual Zone Mini Split yesterday as a DIY. I installed the control wires opposite of the copper lines as well. I simply swapped A and B to match the copper liquid and suction lines(or indoor units) and all worked perfectly. Thanks to OP, I did not think to check this 1st, I went to find a leak and if it was not for this thread I might of evacuated the lines just to try to find a leak. Check the control wires first is what I learned as DIY.

Had the same problem, and this thread helped me solve it! The issue with my unit was that the wiring at the condensing unit was incorrect. Had to swap the comm wires around. Zone one was sending coolant to zone 3, etc. Swapping the communication wires solved it.

Good way to test and troubleshoot is turn on 1 unit, wait 10m, and if it's not blowing cold, leave it running, then turn on another unit. Is the first one now blowing cold? Those are the wires that need to be swapped.

https://www.airconditionersductless.com/

The ductless mini split has many terrific advantages for both homes and businesses: high energy efficiency, zone control, less dust and dirt blown out from ductwork, and flexibility for buildings both new and old.

One advantage that doesn’t often get brought up: because there are multiple air handlers—the small units placed in different rooms around the house—rather than the single one used in standard central air conditioning, if one of the air handlers fails to work, it won’t affect the others. A house or commercial building doesn’t have to lose all its heating or cooling; only one zone is affected.

Of course, there’s still a problem—a failed air handler! If you have an air handler that isn’t sending out heated/cooled air or isn’t turning on at all, you’ll definitely want that fixed. Here are some of the possible reasons for one of your ductless mini split indoor units to stop working:

It’s actually working fine: the batteries in the remote that are the problem

This is a common mistake. If you can’t get the wall unit to turn on, or to changed temperature, the first thing to check is the remote you use to operate it. Put in fresh batteries and see if that gives you back control.

A broken blower fan

Each of the wall units is a mini air handler containing its own blower fan and refrigerant coil. Stress on the blower fan motor can eventually cause the motor to burn out, or it may suffer from an electrical fault. You’ll need professionals to fix this. In some cases, it’s less expensive to have a new unit put in. (There’s another advantage of a ductless system: they’re modular!)

Broken conduits

The wall unit connects to the outdoor cabinet through a set of conduits that pass through a hole in the back of the wall. These consist of refrigerant, power, and condensate lines. If a leak or break develops in any of these lines, the wall unit may either lose power or lose the ability to condition the air.

Frozen coil

Like the coil in a standard central air conditioner, the coil in a ductless mini split unit can freeze over for a number of different reasons, such as a loss of refrigerant or too much dust across the coils. This is less frequent in ductless mini splits (they don’t have as much dust getting into them) but it can happen. If you check on the unit and see a frozen coil, call for HVAC professionals. Don’t try to scrape the ice off, since that won’t fix the problem and may damage the coil.

If you have multiple units failing at the same time, then you probably have problems with the outdoor unit. Don’t hesitate to call for HVAC experts to look into this and fix it.PRO

Hello everyone and especially Cory Show. We are having the same problem with 2 of our 3 units (let's call them unit 1, unit 2, unit 3). Units 2 only cools if unit 1 is cooling. Unit 3 seems fine. May I ask you how many units you have in your system? Also by swapping the comm wires am I not creating a problem with unit 1? Thank you!

To be specific we have an LG system with 3 indoor units

Both my husband and the AC guy initially looked and did not see any problem! They re-checked this morning and sure enough, the communication line was switched between unit 1 and unit 2 !! All is solved for us too.

I have 6 indoor units hooked up to a 48k outdoor unit through a branch box, 5 are working great, however, 1 will not cool, only blows room temperature air??

I have a mini split when the master turns on the slave turns off and will not turn on till you turn off the master

LG master and slave issue here also, I have switched A & B but slave stays slave. I am guessing the installers didn't connect the wires correctly. In one of our other double compressor units without issues, has two black wires (one wire from each unit) together, two red wires together and the two purple ones separate on A & B. The unit that has slave issues has two black, two purple and the reds separate on A & B.