I know this thread is old but I just found out about it because my Tesla service tech printed it out for me. We had this problem recently and it was an awful experience. Because we were in Texas, on a 2+ week trip from Arizona. I have a 2018 MX P100D, bought in March 2018, towing a Bowlus Road Chief Endless Highways. This is a new trailer and we'd only taken one short trip prior to this trip and did not have the Power reduced problem them. This time, we drove approx. 1000 miles, stopping at every Supercharger on our route along I-10 and had no problems. Then, when we were leaving our first camp, I got that Power reduced message. We wasted that entire day trying to resolve it. Tesla couldn't or wouldn't send out a mobile tech but they did tow us to the nearest service: 1-1/2 hours away. But without the trailer, they couldn't reproduce the problem and could find nothing wrong with the car. Best option was to reload the last big Christmas software thinking maybe it was corrupted. We drove back to camp that evening, rehooked up the trailer and drove back to a camp spot and the message was gone, so we thought maybe they fixed it. But...a day later when we started to leave again, there it was. At this point on a Sunday, I had no option other than to drive/limp south to our next destination, where we would stay at an RV park until returning home.
At the RV park, I was able to contact our local Arizona service rep and told him our woes and how on earth were we going to get home? and begged him to help get us a mobile service tech. Which he did (what a Saint!) and several days later, mobile tech met us at our camp and got on the phone with our Arizona rep and the two of them started troubleshooting. They had us hook the car up to someone else's trailer (Luckily just the day before, someone across the way had come over to talk with us about our rig and he was willing) and the Power reduced message came on hooked up to his also. So we ruled out the trailer as the problem. Then he removed a fuse, and the message disappeared! They had us do a test drive, just out the RV park, down the road a mile and back and everything worked fine, including the RF brake controller. What a relief! However....
The morning we were set to return home, all hooked up and ready to go and the RF brake controller would not connect! So I had to drive all the way back with no trailer brakes. Which wasn't as bad as it sounded, with regenerative braking. Tesla service wanted me to bring the car in as soon as possible when we got back, and I just picked the car up yesterday. This is what they think:
To back up, when I first got the trailer, the RF brake controller wouldn't work.
@idoco gave me some tips on what could be happening, and Tesla service came out and switch the wires in the 7-pin and then it worked, as did all the lights. However the way the wires were switched, there is the possibility for the trailer battery to send amps back to the car, and that triggers the Power reduced message. Apparently, as others have said, this design was changed sometime in 2017. Why does this happen sometimes but not always? Perhaps depending on the relative charge of the two batteries? Also, tech informed me the 12V "drive" wire in the Model X is 15A and my Prodigy RF brake controlled needs a 20A circuit so regardless, we are in danger of blowing a fuse. HIs best recommendation? Take the car to someplace like The Specialists and have them run a dedicated 30A cable to pigtail into the pin 1 of the 7-pin connector. My expense of course
For now, they switched the wires back to "stock".
He also said the current set up would work fine with a conventional brake controller but that is not what I have. And the fuse they removed at our camp that 'fixed' the problem was the fuse that allowed communication between car and trailer batteries. Now why the RF brake controller worked after that on our test drive is a mystery.
In addition, 3 times I have had the message come on that disables Traction control, brake control, stability control, cruise control and each time all were restored after the next Supercharger stop. Service thinks this 'may' be related to the other issue but they aren't sure.
