GMC C3 limp mode

[jffmaz]

Hi, 2001 GMC C3, 6.0 keeps going into limp mode. Need help. Its been to numerous legit mechanics, everyone thinks they know the silver bullet fix. Always fixed for a week or so, then it starts again. Has had TB replaced, all all associated possible problems, wiring, wiring, wiring, knock sensors, etc. etc. Been struggling with it for over a year. Nice truck, would like to keep it, otherwise trading it in and gonna stick GMC with it. Dangerous to drive it anywhere but local.

Anyone have any suggestions? It has onlly 130,000+ miles on it and runs fantastic when it isn't in limp. HELP!

[Supreme Pizza]

I am not a transmission specialist, but I assume that a computer locks it into limp mode. If that's the case, there should be a way to scan that computer to find out what the trouble code is. When it goes into limp mode, can you shift into neutral, turn it off, and restart it, and have it shift normally for a while? That method works great on Dodge/Chrysler trannys. 

[jffmaz]

Usually pull over, turn it off and in a few minutes it will run normally. Never shows any codes, even had one guy plug something into the obd port to get data when it happened. As soon as it went into limp mode i pulled it out and it was supposed to let them know what was happening when it goes limp. No codes, no indication of any trouble. Thanks for the response.

[Supreme Pizza]

Might be a loose electrical connector on the trans. Maybe a bad main ground wire somewhere. A standard OBD2 code reader will not show much, except for basic emissions codes. Sounds like you have spent a lot of money replacing unnecessary stuff. Take it to the dealer or a real mechanic and explain that you need a REAL diagnostic, not guesses.