Cars are not very susceptible to EMP. They have to be running to be affected and most cars only stall out and simply need to be restarted.
EMP will destroy devices that are turned off. The EMP pulse will induce destructive currents on electrical conductors, irregardless of whether the device is on or not. Magnetic fields induce current just because it pushes electrons. Electrons and wiring exist in semiconductors regardless of the powered state.