My security light is flashing on in my truck when try to crank it up it will cut on 1998 GMC 1500 Pickup

right off what causes this to happen I do not happen to see my motor size which is 350 vortex

Asked by for the 1998 GMC 1500 Pickup
2 answers
Got another key? If so try it. Possible dirty lock cylinder or key, damaged key, can try turning key on for about 15 min.then off to reset passlock if battery in good shape. May be some info. in the owners manual.
scan anti theft module for codes and post so we can adv.