Need some help with Code
 

I have a 2001 Chevy C3500 HD its a 6.5 diesel two wheel drive. Today while driving the "Service Throttle Soon" light came on and I immediately had a dead pedal after pushing it to the floor the truck started moving, slowly. It was very sluggish the hole time this light was on. I went to Auto zone and had the codes pulled came back P0251. They had no idea for a fix. After I turned the truck off the problem went away and been great since. Any one have a fix or idea. Thanks

How empty was the fuel tank when this happened?

Half tank maybe a little more.

P0251 Injection Pump Cam System

Most of the time P0251 mean bad IP but i see you said it clear up could have been some dirt in it or maybe PMD on it way out

I do LP test first

Here some info i found on it maybe some help

You have to zoom in on it to read it

Also check all plugs and clean at APP