Q: 2000 Chrysler 300m will not start because of the anti theft system

You can refer to YourMechanic’s instructions to de-activate the engine immobilizer system. You should also consider the possibility of a weak battery, excessive voltage drop in the starter circuit due to poor electrical connections (be sure the ignition switch was fully reconnected and nothing was missed during the repair), or a dead starter/solenoid. Prior to performing diagnostics the battery should be load tested. With a battery in good condition and the immobilizer (security) system off, if the starter motor does not operate at that point, there are only two possibilities: 1) either there is no power to the starter motor/solenoid, or an excessive voltage drop, hence the entire starter circuit (again, including the ignition switch) would have to be diagnosed; or 2) there is power but the starter/solenoid itself is bad and that includes intermittent operation which does happen. If you desire that a specialist from YourMechanic determine which of these is the issue, and get it resolved, please request a no start diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.