Question

2001 Cadillac Catera with code U2105 and the engine won't start

Answer 1

U2105 will be set if the TCM has lost communication with the ECM, Each of these modules send information high , low  pulse with each other over a can bus line, If the TCM has not received ECM data for a specific amount of time DTC 2105 will set, Loss of communication can be from the can bus line being open, short to voltage, to a faulty ground on either module

U2105 Cadillac Code - Lost Communications with Engine Control Module
Read more: https://www.autocodes.com/u2105_cadillac.html