2002 Cadillac STS has new battery. Security and theft system come up on screen to be checked, its not a constant thing. The car will not start, had it hooked up to mechanic computer and found nothing. Could this be a motherboard problem? What are your thoughts?

thoughts are INSUFFICIENT ground potential...from your battery or is it batteries...ya the black cable connects to the chassis/ground....right now you're on a "brown" connection...there are varying available amps and varying volts there now...you MUST take a wirebrush...take the connecting bolt off the lug at the end of the black 1/0 battery cable...wirebrush the lug holding it with a pliers so as not to damage it....then scrub the chassis until metal is realized....with this restored grounding to the components such as the fuel pump and ignition systems....oh and the greedy starter motor who's created this corrosion with the dis-similar metals pushin' 300 or more amps through the single spot there....If you're lucky there will be a second 1/0 black cable goin' to near the alternator...the block is where the starter is~