2004 LLY duramax 6.6L TCM lost communication with ECM

Scotty, I need some help bud. I have a 2004 Duramax and the TCM has lost communication with ECM. Codes P0700 and U0101

It runs and drives good, only issue is around 35 mph in 3rd gear, but its like it shifts into 5th after/ limp mode?? But then 5 sec later it shifts down to 4th, on to 5th and everything is good. Also add it doesn't do this every time I'm going through the gears.

I have checked pins on TCM and ECM on this year its one module to my understanding. I looked for wire rub on every wire i can follow. People say could be open network in CAN bus. Cant find anything online that shows me where this is on my truck. I replaced the NSBU due to P0700 code said that is the culprit. Checked the grounds i could find. cleaned them up with water and baking soda and cleaned the metal, then reattached them. Replaced the input speed sensor on the trans. I have a wheel counter i need to still replace so not sure if that will fix it.

I have been watching your videos for a couple years now and know you do things right! If you can help me or know anyone that can help i would greatly appreciate it. I look for stuff online and cant find my issue anywhere. This has been going on for several months now. I do drive this on the daily about 40 miles per day and haven't pulled anything with this issue, as i am afraid this will cause more damage. I have taken this to 3 different shops, 1 was a transmission shop where i know the owner and he was honest and said this could be a 30 min find and fix or 4-8 hours as well but said the transmission is fine and its an intermittent problem.

Again thanks for the videos and if you have any idea please let me know!

Thanks, Jordan

Comments (1)
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working on those computer bus lines, you really need full wiring diagrams and a dealer level scan tool to analyze modules for correct communication and finding of flaws. Otherwise, good luck ohming the resistance on all the wires and connectors, and realize you HAVE to do dynamic loaded testing of them, not just static test with a volt meter