My sons truck is having fuel mileage problems. One day it's getting 20 mpg the next its 14mpg with black smoke. Code is P2509 truck is 3/4 ton auto trans, bullydog program I think.

I would unplug the box and see what it does then.

Here is what the service manual has to say. [ATTACH]3232[/ATTACH]

P2509.pdf

So just check conections?

How old are the batteries? Weak batteries can cause issues on a CR.

I just talked with him and he knows they came on the truck 3 years ago. Getting load tested and hope thats all it is.Thanks for your help