My Cruz is throwing a u100 code that code say its failing to talk to pcm . i

U0100 will appear for certain categories as defined but he diagnostic tool used. I would advise obtaining a diagnostic tool that has the ability to read and share more complete information, or referring to a technician with such equipment that can do so. Learning what area the U0100 code is setting will possibly help determine what is going on. On the other hand, a "no crank no start" issue is usually not going to have helpful codes associated with it. Troubleshooting should indeed start with battery testing and analysis - is it fully charged and able to handle a load? Then sure, check and ensure all battery cabling is properly secure and free of corrosion. Do dash lights come on when you turn the key on? So much as far as basics to be done here before you get into codes. U0100 could be due to low battery voltage and a lack of communication as a result, for example, and may require no troubleshooting in itself.