Problem with my 2001 Chevrolet Malibu.
V6 3400
dash lights = security theft light
Repair work done prior = Replaced the ignition cylinder because the key would not turn, and
now it wont start.

Question = Why does my chevy malibu not start after changing the passlock cylinder. The key would not turn so my husband replaced the cylinder. The theft security light will come the dash. I have seen on other sites that there is a re-learn code procedure to get it to start. How do i fix this problem. Are there any other parts involved, or just a code relearn.

Here is the
relearn steps you needed.

Passlock ignition cylinder.  The relearn procedure you are referring to is a temporary fix. This will usually get your car to start again, but eventually will fail to start again.  Replacement of the cylinder is required.
Chevy Malibu Theft Learn
2004 Chevy Malibu  Security light on.
We changed ignition switch , change key cylinder with chip, and body module.
Question = Done all the above, even the passkey cycling method and car still starts and stalls.

Assuming you did a theft relearn after replacing the ignition switch and Passlock cylinder- but that did not work, then you replaced the BCM, we need to know if you programmed the BCM for your car? If you did, did you then do the relearn again? If not, this is your problem.

If all this was done properly, then you must have a wiring problem between the cylinder and BCM. Also, if the car has a non-factory alarm or remote start, you need to get that out of the system, as these are known to cause problems with the factory security system.

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Here is that procedure.

Keep a battery charger on.

1. Turn the key ON.
2.Wait for 10 minutes or until the theft / security light goes out.
3.Turn key off for 10 seconds.
4. Start the car. Learn is complete