car power goes dead

I have 2 buick rendezvous and am having issues with the car power shutting off. I have no power whatso ever, entire car goes dead even when driving. In one car the lights won't go off and has codes flashing off and on. Took to auto zone and put the computer on it and it showed no codes, until I got ready to leave then service engine light came on but went off when I flashed the bright lights.:screwy: Has anyone had this problem? Thanks