Hello!
We have two first gen Mazda 6 S 3.0L V6. Both cars drives fine but the AT + CEL Light will come on from time to time and goes into limp mode. Both cars also have the U0100 code.
I am not sure if this is related, but it seems that this happens very often when we park in an slight incline.
If you shift the transmission to park, pull up the handbrake and shut off the engine without letting off the foot brake, the car will move a bit. This will make the car jolt the next time you start the car and you shift it out of park. This often triggers the AT light. We have to shut off the car and restart it again.
If you shit the transmission to park, pull up the handbrake and let off the foot brake before shutting off the engine, everything goes smooth. We can drive both cars for a very long time without problems until our procedure of shutting the car off in inclines is “wrong”.
Aside from the codes. I apply the parking while the car is in gear, then take my foot off the brake with the car IN GEAR, this ensures the parking brake is holding. Then I put it in park.
U0100 code:
This is a generic network communication code, which means it covers all makes/models, 1996-newer. However, specific troubleshooting steps will vary depending on the vehicle. The generic OBD trouble code U0100 is a serious situation where the signals between the electronic control module (ECM) or the powertrain control module (PCM) and a particular module have been lost. There could also be a problem with the CAN bus wiring disrupting communications.
Read more at: https://www.obd-codes.com/u0100
Copyright OBD-Codes.com
Hum… but we have several vehicles in the family with automatic transmissions. A 2002 Lancer, an 2007 Sienna, a 2014 Corolla and non of them jump and goes into limp mode like the 6’s when we park “the wrong way”. 
These two cars drive without problems. The transmission shifts smoothly, no funny noises or slipping.
Also both cars have the same code… How do you troubleshoot something like this? 
Frankly, I don’t. I leave that to mechanics that have the diagnostic tools and knowledge to diagnose the problem.
I would take both of those and make a deal on a new warrantied vehicle that did not drive me crazy. You seem to have several threads about problems with one or both of these . Life is too short to let a vehicle be that much trouble.
The only thing I can come up with, and it’s just a guess, is that somehow there’s some stress on the parking pawl in this situation, and that’s transferring to some other gadget or wire harness in or near the transmission, causing whatever it is to malfunction. Ask your shop to put it on the lift, have someone inside shifting, stepping on the brake, engine running vs not running, etc while someone underneath the car is looking for some sort of mechanical interference happening. Pay special att’n to the linkage from the shift lever to the transmission, as there may be a cable with a faulty connector that’s getting displaced when that moves. I had a misfire problem on my truck b/c of that exact situation one time.
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Alright, I will try that first but I find it weird that both cars has the same problem. Thanks for the suggestion. 
Lol I agree but I can’t sell them because non of these two cars are mine.
@Triathlor You say we have - then you say they are not yours . Not really clear .