U1255 code
 

I'm hoping that someone has suggestions to a problem that I am having with my 99 Seville STS with 98k Miles. The car will not start but will crank and my mechanic has spent alot of time trying to figure out what is wrong but stll has not found the problem. He has no communication with the main computer and he gets U1255 code from the other modules. He tried swapping the main computer but there was no change. Any ideas? thanks