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’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.