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I recently purchased a 2001 Chevrolet Silverado 2500HD Crew Cab Long Bed 4x4 with the Duramax and Allison. I was driving through town and I noticed all gauges lost power except the oil and volt gauges, and the truck was acting like it was in neutral. I coasted to a parking lot and shut it off and turn it back, still having the same symptoms(btw the whole time the engine ran normal). I called my local mechanic and he says it went into Limp Home Mode. After waiting 30 minutes I started it up and still the same, I was able to get it in gear and only stayed in first gear the whole way to the shop(because Limp Home Mode). He put the scanner on it and it was showing Transmission Communications Lost. Has this happen to anyone?
 

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The TCM went bad, or the wires are possibly frayed............look at where the wire bundle attaches to the transmission
 

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So we tested the TCM and that's okay. We ending up replacing the harness. But now the transmission is acting like it's in neutral, and not even going to Limp Mode.
 

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So after replacing the transmission harness, next we replaced, the ignition harness. That temporarily fixed the issue for about 20 minutes and went back into Limp Mode. Noticed there is "play" in the ignition slot for the key. Ordered a new tumbler for the ignition and new key. As of right now it seems its been security issue all this time. Has anyone dealt with an similar issue or what's your opinion?
 

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I have had this issue several times and it is so intermittent that I can't seem to get it figured out. My volts and the oil pressure , speedometer and tachometer keep working. Security light on battery light on air bag and abs light on. Gear selector is blank but no limp mode. I cleaned my battery connections and checked my grounds. It flashed for about 10 seconds the other day again and went away. The other 2 times I had to pull the battery cables off. I have had some u codes last spring, but it went away in 10 seconds. I forgot about last spring so I thought it was a fluke. I just remembered that. Maybe that's my problem. Keep me posted I curious before I start throwing parts at it.
 
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