1998 Chevy Truck

Hi there!

The P1631 has to do with a communication error between your anti-theft system and your engine computer.
When you start your engine, the ignition switch/key enable your theft deterrent system to send a "password" to the engine control computer - basically letting it know that the truck is being started with the proper key - not being stolen. If for some reason the proper key is not being used or the truck is being started by some other method, it will send a DISABLE password instead and the engine computer will cut power to the fuel pump so that the truck cannot be driven.
Firstly I'm curious if you are using the original key to the vehicle? If not, that may be causing an issue.
Unfortunately proper diagnosis of this problem requires the use of a diagnostic scan tool and a mechanic that is trained to use it:)

Hope that helps. Good luck!

Keren