Hey, everyone, I'm having a similar issue, and thought I would tag on here to see if I can get some thoughts and feedback. I have a late 2017 MX 100D with a gen 1 UMC (40 amps). Since my purchase in Dec 2017, I've been charging using my UMC on a NEMA 14-50. It's dedicated run of about 15' of 6/4 to the main panel to a 50 amp breaker.
I've never had any problems getting a full 40 amps until about a month ago when I started to get the "check wiring" error and the light ring on the charge port would never change from dark blue to green. Also, my nightly scheduled charge often fails, and when I can get it to charge, I'm only getting about 23 amps. This has left me in sticky spots a couple times.
I contacted TCS and they ran some diagnostics and couldn't find any issues, and told me that the UMC may be faulty in a way that they can't see via the diagnostics. They also had me try the UMC using the 120v plug, and that had the same communication issue (dark blue charge port ring remains). I tried charging my MX via other AC chargers (J-1772 and Tesla Destination Charger) and it charged at full power as expected. So, I took my MX and UMC to the TSC, and they tried it there on their NEMA-1450, and my UMC worked JUST FINE! So, they decided it wasn't my UMC.
Additionally, right now I have an MX loaner (my MX is at TSC for software updates failing due to bad modules, including the AP module, so fingers crossed it gets replaced with the new FSD module!), and the loaner has a gen 2 UMC... and... I'm having the same issue with this MX and gen 2 UMC. Only getting 32 amps.
So, the sensible conclusion is that something in my wiring is causing the issue. The baffling thing is that nothing has really changed in home (certainly not the wiring), so I can't figure out why it has just now started giving me issues. The only change is a new fridge, but it's a new model with same amperage requirements, so I don't see how that would be causing any issues. And my solar hasn't changed, but I'm charging at night, so that shouldn't be adding any noise to the system. I guess it could be a utility issue outside the house that is throwing spikes, but that's just a guess. So, I guess I'm going to have an electrician check out the wiring to see if they can find any issues... perhaps it's a lose connection or nicked wire.
Thanks for reading my novel. Would appreciate any thoughts or possible solutions!
Cheers