2001 Chevy Impala Theft Relearn.
Security Light Flashes
on and off. Car wont start.

Repair done prior = The ignition  cylinder will turn but the engine wont start. I  have seen there is a procedure to re-learn the theft system. How do i  relearn codes so car will run.

Question = The ignition was changed to allow the key to turn. Now it turns but will not start up and the Security Light Flashes. Looking for step by step instruction on how to reset the Security Codes which will allow the car to start. No other problems just the security wont let the car start up.

Thank you for the tip/donation. Here is the procedure you requested.....

Turn ON the ignition, with the engine OFF.

Attempt to start the engine, then release the key to ON (vehicle will not start).
Impala Security Relearn Procedure

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Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF.

Turn OFF the ignition, and wait 5 seconds.

Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes (the vehicle is now ready to relearn the Passlock Sensor Data Code and/or passwords on the next ignition cycle.

Important: The vehicle learns the Passlock Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.

Start the engine (the vehicle has now learned the Passlock Sensor Data Code and/or password).
With a scan tool, clear any DTCs if needed (history DTCs will self clear after 100 ignition cycles).
Be sure to have a battery charger on the car, as this 30 minute procedure may drain the battery to a point were it will effect the programming- or cause a dead battery.

The other way is to have a dealer scanner re-program the system.