I have a 2006 trailblazer 4.2.

That's a security system problem that needs to be repaired by your local Chevrolet Dealer. Hope that helps! Jim

Try a security relearn. A Case relearn needs be done by dealer. Just copying and pasting instructions. 1. Attempt to start the truck by turning the key to the Start/Run position. Try to crank the vehicle. 2. Observe that the vehicle fails to start and the security light is illuminated. 3. Release key to the ON/RUN position (not the ACC position or OFF) 4. Allow the vehicle to sit with key in the ON/RUN position for no less than 11 minutes. Security light will turn off after this. 5. When the light goes off, turn key to OFF position and allow the vehicle to remain in the OFF position for no less than 30 seconds. 6. Repeat #1-#5 for four cycles or until vehicle starts.