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Car needs service, “power reduced” alerts [Update: fixed, loose main ground connection]

I was driving along coming up to a red light. Regenerative braking was doing its thing. Then all of a sudden, the car started to shudder to a stop. It was like the pulsation of anti-lock brakes. It was a bright sunny day with dry roads and 88 degrees. The car showed P like it was in park, and a bunch of error messages started cycling on the screen. Car needs service, power reduced, charging not possible, automatic emergency braking disabled, etc. There may have been more. I don't remember. My dad was with me. He was able to roll down his window and open his door. I could not. The air conditioning had shut off. Yes, I was able to use the manual override to open the door because it was getting hot very quickly in the car. I called Tesla Roadside Assistance. They had me try to reboot the MCU. That did not work. In the meantime, my dad was able to push the car with great effort to get us out of traffic. Apparently the car wasn't really in park. I was able to steer the car.

I got the text from Tesla that the tow truck would arrive in 90 minutes. While I was waiting, the car slowly started to come back. The seat and steering wheel would start moving like going between Easy Entry and my driver profile. I was able to roll down my window. Next I tried to reboot the MCU. It worked that time. I was able to turn on my 4-way hazards. The error messages were still coming up, although different ones. Parking brake not set; Car could roll. The traction control icon was also on for a while. I still could not get into Drive. After two more times of the car changing driver profiles on me, the car came up like normal with no more error messages. I was able to put the car into Drive and move it a few feet to get to a more optimal safe position away from traffic. I called Roadside Assistance again in case they had done something remotely to reset the car. They said they did not (and could not). They recommended that I still wait for the tow truck.

When the tow truck came, he was able to drive the car onto the flat bed and (I assume) get the car into park like normal and have the parking brake set. The flat bed was still in a ramp position when he got out of the car and the car did not roll back.

And now my car is parked at the service center.
 
Good news. Got the car back already. Car was great on the drive home (back to work) 37 miles.

From the work order:
Found alerts indicating loss of communication to multiple modules. Inspect connections at left vehicle controller. Found loose main ground connection. Secure connection. Perform test drive and function test of vehicle systems. Vehicle operates normal.

I guess that explains why my dad was able to open the passenger door, but I could not open the driver's door.

The technician said that the shuddering was from the car unable to handle regenerative braking correctly.

One of the things I forgot to mention in my post was that there's a bump in the transition between asphalt and concrete right by the intersection. Hadn't thought about it right away. Been driving over that for years. Don't really think about it.

They also updated the firmware to 2019.32.1 from 2019.32.

I didn't get any nags about holding the steering wheel from Autopilot. I was getting them every 30 seconds or so while holding the wheel previously.
 
Haven't happened to me personally but one of the M3 owners I follow on youtube had this happened and he simply hard-rebooted the car (press and hold steering wheel nipples while pushing the brake pedal) and everything came back to life. Computers....o_O
While it would be fun to blame computers, the ground connection is not computer related, and it is installed by hand by a human.
 

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