* Loss of Memory: When the battery is disconnected, the car's computer loses its memory, including information about the registered transponder chip in your key (or the security code associated with it). The anti-theft system, sensing this lack of registration, activates and prevents the car from starting. The flashing light is the visual indicator of this system being engaged.
* Security System Reset: Many older systems require a specific procedure to re-register the key or reset the security system after a battery disconnection. This might involve a series of key turns in the ignition, waiting periods, or even using a specific diagnostic tool. Simply connecting a new battery isn't enough.
* Faulty Key Transponder: It's less likely, but possible, that the battery disconnection revealed a problem with the transponder chip in your key fob. This chip communicates with the car's anti-theft system. If it's malfunctioning, the system won't recognize it.
* Security System Malfunction: In rare cases, the anti-theft system itself could be malfunctioning independently of the battery change. A faulty sensor or module within the system could trigger this issue.
What to do:
1. Consult your owner's manual: This is the first and most important step. The manual *should* have instructions for resetting the security system after a battery disconnect.
2. Try the key re-registration procedure (if the manual specifies one): This typically involves a series of key turns in the ignition, often including the "on" position but not starting the engine. The exact procedure varies by vehicle, so refer to your manual.
3. Have the car towed to a qualified mechanic or dealership: If the manual's procedure doesn't work, a mechanic can diagnose the problem using a scan tool to communicate with the car's computer system. They can determine if it's a simple key re-registration issue or a more serious problem with the anti-theft system itself.
Important Note: Don't keep trying to crank the engine repeatedly with the anti-theft system engaged. This could drain the new battery quickly or even damage the starter.
