Why does my 2001 impala flash security and car not start?

This is usually caused by a faulty passlock sensor requiring replacement of the ignition lock cylinder. To test:

Crank the engine momentarily (like you're trying to start it) then allow the key to return to the on/run position. Your check engine light should be on, and the radio should work, but turn the radio off if it's on, because you need to keep the battery charged.

Wait fifteen to seventeen minutes, then shut the key off and remove it. Then wait fifteen seconds (or longer) and start the engine. If it starts, you need a new lock cylinder. With the engine still running, press the key straight in. If the car stalls, you need an ignition switch also.