2002 L300 Limp Mode
 

Hey,
This is my first time here, I've found many answers on this site but this problem continues to puzzle me. Let me start by giving a little history for the car. It was originally my dad's, where he maintained it and it ran fine, then he gave it to my brother, who still took care of it but it's where issues (like limp mode) started happening. He has replaced the MAF Sensor, the BCM, the thermostat, had the transmission fluid changed. Eventually the car wouldn't start after running for a while, that's when he stopped driving it. Then last summer he told me if I could fix it I could have it. I talked to my friend's dad who is a mechanic and we replaced the crankshaft sensor, and that fixed the starting issues.

The limp mode (the service light comes on while driving and it starts to shift hard) continued, but I lived with it (while looking for an answer) much like my brother did. However, recently it has gotten a lot worse. Now if it has been running for 10 minutes or so, then I turn the engine off and try to start it again it starts, the light comes immediately on and it won't let me shift for a good 15 seconds, then when I am able to shift the engine won't go over 900 rpm (and it limps in short bursts). I have replaced the TCM to fix this, but I don't know where to go from here.

Codes:
P1780
Manufacturer Controlled Transmission
U1206
Manufacturer Specific DCT

Thanks for everyone's help!

U2106 Lost Comm w/TCM.
It is a BCM code. So BCM not communicating with TCM. That would indeed be a problem as it doesn't know when to shift so it doesn't , to protect itself.
Would guess it is a loose damaged or corroded contact at a connector or a damaged wire in the Communications bus

P1780 Transmission control module TCM requested MIL illumination
Standard procedure when TCM has issues -- it sends a P code to the BCM requesting the Service Engine Soon light to go on.
I find that interesting it the TCM could throw this if it cannot communicate w BCM. Anything's possible. Ignore for now.

You'll have to track down the BCM to TCM "disconnect" to remedy this.
My guess anyway