My check engine light is NOT on but P1689 "No Communication Between ECM & Injection Pump Module" shows up when doing code trick. Will this effect my gas mileage how do i fix it??

I must assume that your truck is currently running. If so must be an old code set for some reason maybe there was work done on it a a previous time. If you had your vp44 fail in the past maybe the code wasn't cleared. But if you truck is running it should be fine. If the code was active your truck most likely wouldn't run at all

i've seen that code with my old Comp with two fixes. One was to clean the ground (and add a second ground to the left front fender where an existing battery ground is attached) and the other required the box to be sent back to Edge for repair. Annoying, but they fixed it free after I'd owned it for several years... Both times the truck would stutter and buck, so if your truck is running OK, I'd agree that it is probably an old code.
Where are you located? Find a local to clear the codes with a Smarty or ther DTC tool. See if it comes back.

I'm in Oregon