98'Malibu can't start

My 98 Malibu(4 cyl, 100K mile) can't start now. When I turn the electricity on, the fuel pump relay switches on and off quickly, making a lot of noise, while the fuel pump doesn't work. The relay itself is good. Checked the voltage at the fuel pump, only 5V instead of 12V. I took the car to an autoshop, they ran a computer diagnoise and the code read out is something like "communication problem between module and computer". The guys in the shop said the code is comfusing and have no clue now.

It looks like an electricity problem but not the famous passlock one. Is there anyone has the similar problem with Malibu before? Thanks.

just to be safe, swap relays with a known good working one...horn or whatever. you might have probed the sender or pressure sensor wires and those are supposed to have 5V going into them. pump itself can have internal short, could be blown fuse as well. let us know what you find