Yesterday morning I plugged in my 2020 Clarity (11900 miles) in the garage (120V charger). An hour or 2 later, I went out and unplugged it (it was about 70% charged). It was cool in the garage, 65 or 70, and while charging it was not making any excess noise. Right after I unplugged it, the engine fan under the hood came on like a jet engine. I've never heard it so loud before. After fretting about it for a couple minutes, fan still running, I tried plugging it back in. The fan immediately stopped and it went back to charging. Another hour later I repeated the cycle: unplug, radiator fan immediately going into overdrive, wait a minute, plug in, fan stops and goes back to charging. At that point I unplugged it (fan came back on again), got in, and tried to start it. It wouldn't start and flashed various things on the dash -- it thought I was in accessory mode, it wouldn't let me change gears. (Although I think the fan did cut off, maybe when I first tried to start it.) It kept saying "Accessory mode -- press Start twice to turn off" so I kept trying that. It also started cycling through every error known to man (Tire Pressure Monitor, Brakes, Power System (?), etc). Eventually I was able to turn it fully off again, and then it sat quietly in the garage as I prepared to take it into the dealership. When I was ready to take it in, voila it started ok and I was able to put it in gear. However it did go through a litany of errors on the dash again (Tire Pressure Monitor, Brakes, etc) that continued as I drove it to the dealership. The check engine light was on as well. The dealer said that they can't find anything actually wrong with it. Their printed report says there were "a lot of miscommunication codes related to battery condition". They cleared the codes and it seems to be fine now. They noted that they found 2 cases of the same problem reported on Tech Line Forums, and that in those cases the problem did not recur. Hopefully this is a once off problem that will not happen again. Seems like basically the computer needed a reboot, so to speak. I was curious if something like this has happened to anyone else.
When my car had a similar brain fart I disconnected the negative cable to the battery for a minute then reattached. Got lots of error codes then it worked fine.
If the charging cycle doesn't make good connection, it can constantly try to reconnect which can kill the 12v battery. At least that happened to my old Fit EV. As noted above I'd disconnect the 12v and reconnect, make sure it's charged, and see what happens
Just to amplify the point - When you disconnect / reconnect (or if the battery gets discharged or fails), it will do a type of 'reset', and you WILL get a Christmas tree of lights / failures on the dash when you start. You have to drive for a while before these clear (maybe several miles). All these errors seem ominous but it is apparently 'normal' for the Clarity. It can give the false impression that might be unsafe to drive the cehicle. It sounds to me like you may have experienced a discharged battery as part of your experience. If the car sits idle for a long period (couple of weeks), or if you only make short, infrequent trips, it is possible that the battery became discharged. If you added some accessory that uses a small amount of power even when the car is off, that will speed up the discharge.
When your computer/phone crashes or acts up, 99.9% of the time you just shut it down and restart and all is well (in fact, Android asks you to perform a restart occasionally even if all is well). As the Clarity software is stored similarly, it's really not surprising that the same process probably applies. I do the restart every once in a while when I notice reduced responsiveness or if a glitch occurs. So far, it's always fixed the problem.
