Can someone please explain what "limp mode" is on a Duramax? I see that mentioned a lot and I have no idea what that is?

The triangle is a warning lamp and limp mode means it could be a serious, (expensive), problem. You will need to get the vehicle plugged into a diagnostic system or an OBD2 scanner to get the trouble codes. Most Good mechanics can do this for a Small cost, dealerships will charge a lot more. These trouble codes, when translated, will help lead you to what the problem is. The code will be one letter followed by four numbers. Comment below with these codes and I'll try and help explain what they are.

https://www.youtube.com/watch?v=KOP0V31oqoQ
https://www.youtube.com/watch?v=JjqBnj2tbr4
https://www.cargurus.com/Cars/Discussion-t10839_ds516125
https://www.cargurus.com/Cars/Discussion-t32618_ds701474
https://www.atptrucks.com/articles/duramax-and-allison-limp-mode

Does the transmission have an over heat switch? That is the only thing I can think of. If so, you need to add another transmission cooler to assist the one you have for those heavy loads.

Does the yellow engine management light come on during this? If so you should get someone to check the fault codes as there are several reasons for limp moding.fuel lines , electrical system, low oil pressure, overheat issues etc.OD11 testing will save you lots of time and give you a direction of where to put things right.

no it wont, it is most likely a transmission slip setting the limp mode. better save for a new allison

try to remove filter and pump out fuel from the tank. the gunk you loosened with "mechanic in a bottle" goes to filter then when ENGINE IS SHUT OFF GUNK SETTLES BACK IN LINE. NEED TO PUMP IT OUT TO SOLVE PROBLEM

mo me it sounds like low fuel volume i would check my lift pump

sensor for boot gone away simple job in the boot lock is located or next to it about engine explain wat s limp mode? is not starting at all?