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I installed a mini-split system in my shop (it's the second mini-split I've put in; the first went great). Everything appears fine, except that after a few minutes, the system shuts down and the indoor unit displays "CH 05." Googling, this turns out to be a communications error from indoor to outdoor unit (I haven't yet checked the status on the outdoor unit to see if it's two-way). I found a diagnostic manual that had me check some voltages and resistances along the cable, and it seems the signaling is occurring just fine. Nothing is shorted and voltages are within spec, and change as described. The diagnostic procedure ends there with no additional recourse.

Anyone have any experience with these, and have any suggestions? The run is only 25', and the LG manual doesn't worry about EM effects until 130'.

I'm gonna put an oscilloscope on the comm line to see what it looks like. Maybe it's super noisy for some reason.

At this point I'd guess they'd just swap out the PCBs on both units, but getting them to do that for me under warranty is going to be challenging, since I'm not a "qualified" HVAC technician (I am, however, an electrical engineer, and not (always) an idiot.)

Update: Well. I decided to swap the L1 & L2 connections, thinking maybe there was some wonky issue with coupled inductive noise, or who knows. But now it’s working. The only other thing might be this: I have an extension cord going through the same hole to provide power for tools and lights while I was working on the outdoor unit. That is currently unplugged. Now, I unplugged it when first testing, just in case that small amount of proximity was the issue and it had no effect. So it seems swapping the AC lines were it. I may swap them back to see, but for now it's nice to be heating my office!

If anyone has more advice on troubleshooting this issue, please feel free to answer below.

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  • Being an engineer you should have known that there would be no difference with swapping L1-L2. I would have suggested verifying that the com wires were snug with a tug test on the wires. I run the com cable(s) with supply wires on most installs the extension cord should not have affected the com. Did you install the required 120v GFCI protected receptacle? At least under the NEC there is a requirement asking since you had a cord there.
    – Ed Beal
    Feb 15 at 15:59

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