I’m a little lost on trying to trouble shoot this one, any advice is appreciated. I purchased a 2 ton single head Daikin minisplit system online a few months ago to install in a workshop in my backyard, that I’m in the process of renovating. Everything was going smoothly with the install, doing the majority of the work myself. I am a builder by trade and have done several minisplit installs over the years. I hired an HVAC tech to connect the refrigerant linesets, and evacuate/ pressure test the lines. As far as I can tell the problem came about when I connected the wiring. Unfortunately I crossed the signal line and one of the power lines at the outdoor unit control wiring. When I went to fire up the system, the indoor unit status light started flashing blue, indicating a system error. I realized what had happened then and corrected the wiring. No luck, still flashing blue. Error code retrieval at this point, at the remote control reads U4: communication error. Next I tried pulling the outdoor unit control panel to see if there is a blown fuse. I was able to locate 3 separate fuses but none of them tested bad. Pulled the indoor unit control board and there is one fuse which also checked out ok. Last I try running a separate jumper wire externally to eliminate the possibility that there is an unseen break in the original signal wire. I then try turning the system on again, same thing: indoor unit status light flashing blue and error code U4. Any ideas?

I’ll try to keep this as brief as possible, I’m trying to resolve a U4 code on a single zone RXL09QMVJU, I do not currently have the model handy on the indoor unit. I know the system is old but I’m with one of the few companies in my area that actually repairs things unless a replacement is absolutely necessary.

I’ve been on the phone twice with Daikin tech support, I’ve run through all of the diagnostic procedures they’ve given me so far and even run through others that shouldn’t throw this particular code. At this point both indoor and outdoor boards have been replaced and I’m starting to question if I might have a board that was DOA when I installed it.

Indoor unit turns on, will run on fan mode but any mode that would involve the compressor errors out, I have proper ac voltage to everything, per their procedure Daikin says I should be looking for 53-55v dc at the indoor board from terminal 3 to ground with the communications wire disconnected. I have 75v dc, I also have the sister unit ten feet away in the next room running fine with 75v dc at that indoor board.

Outdoor board sends no voltage to the compressor, fan was ohmed out and fine, emergency cooling runs the indoor unit at full capacity but again outdoor unit doesn’t run at all and I have continuity in the wiring between indoor and outdoor units.

I’ve been getting some inconsistent information from Daikin between the manuals and talking to them so I figured I’d ask here for opinions.

I had a Daikin Minisplit System installed in April of this year. About a Month ago the system gave us error code U4 and stopped working. I turned off the breaker turned it back on and it worked fine. I figured a fault must have happened somewhere and that it would be fine. Today the system does the same thing but now if we try resetting it nothing happens.

I called the company that installed it, and they stated it must have been that we used auto mode. They're coming to look tomorrow, but I guess I'm wondering if any other technicians have noticed the same issue? I read the manual after installation and understood all three indoor units needed to be the same mode. It seems odd to me that having it set to auto would cause a failure at this level.

The indoor units are Daikin model FTXV12AVJU9

My air conditioning failed on me some time today when I was at work, and when I returned home it was 92° inside my home. Figures this would happen during the (hopefully) hottest week of the year in Los Angeles. This system was professionally installed in June of 2020, and has worked without issue until now. The company that did it was bought out by another larger firm, and they are unable to send a tech out for another 17 days. I’m kind of SOL there. I change the filters every six months in the air handlers, and regularly hose down/clean the exterior unit. With a basic visual inspection, it does not appear that there are any blockages/impediments to air flow. I have tried powering off both the exterior unit and both interior air handlers by switching off the breakers on two separate occasions. The cooling works for about 10 minutes and then the error message is displayed again on the interior control panel. Any ideas?

I have a Daikin Minisplit with 3 heads operating off it. It’s been working great and keeping up nice and cool. It cooled off here for a few days, so I decided to turn all 3 heads off. The temps shot back up yesterday and now none of the heads will run on any mode. They all attempt to start but then go to fault mode. The fault I’m getting is U4. I understand that this is a communication/sensing error and most of my research shows that this is a wiring or board issue. The fact that it was working fine, I think I can rule out improper wiring. Any suggestions? Thanks

Living in a house built 1995 with an aircon unit probably just as old. It’s a split system Daikin that was working but now shows an error U4 and will not turn on. A new system is so expensive I wanted to see what’s the likelihood of this being fixed. I’ve tried turning on/off with no luck. Not keen on paying a specialist just to say it all needs replacing. Anyone had this issue & been able to fix it?