I ran into this problem over a year ago with our S P85.
Our garage has an HPWC on a 100A circuit - and a 14-50 outlet.
When the car started having charging problems on the HPWC, I tried using the UMC on the 14-50 outlet - and ran into the same charging problems - charging would not start, or if it started would stop after a few minutes.
I even tried using a 110 outlet - and had the same problem.
I called Tesla - they looked at the logs - and they believed the car's charging system was OK.
The utility ran there standard power check - and we were getting the correct voltage.
It turned out, other Tesla owners were having the same problem within several miles of our house.
After getting Tesla and the utility to talk to each other - the utility discovered problems in their distribution equipment in the area - which was introducing some noise on the power lines. It wasn't enough to disrupt most electrical devices - but was enough to prevent the Tesla chargers from operating reliably.
It took almost a week until they were able to do a trace through the grid equipment - and find the cause and repair it.
NOTE that when this happened, the normal power technicians probably don't have the equipment to test this. They had to install a special logging device on our circuit to do a more detailed analysis.
If you're seeing charging problems on both UMC and HPWC, my advice:
- Take your car somewhere else and try to charge using your UMC or a public J1772 or destination charger. If that works, then you've probably ruled out a problem with your UMC or onboard charger(s).
- If you know any Tesla owners in the area - check to see if they are having issues. If not, take our car over there and see if your car charges.
- Call Tesla's phone support to have them look at the logs for your car, and see if they find any charger warnings or faults.
- Call the nearest Tesla Service Center to see if they are having reports from any other Tesla owners in your area
While it's most likely a problem with your onboard charger(s), it's also possible you may have a problem with the power inside or outside of your house - and it may be more complicated than just verifying voltage levels.
WARNING - Tesla assured me the car's charging equipment was protected from damage due to power problems or surges. But, despite that, the secondary charger on our S P85 (which had the dual 40A chargers) blew out during this time. This was covered under our extended warranty - so it is possible for bad power to damage your onboard charger...