2000 Chevy Truck security light

You can try a "Passlock Relearn" and see if it will take. If this doesnt work youmay need to have the PCM/VCM sscanned to see where the communucation is being lost in the security system

GM PASS LOCK RELEARN- Ignition to "run" position, try to start and let key return to "run" position
Approx ten minutes security light will go off
Turn key "off" and wait 5 seconds
Repeat 2 more times for a total of 3 relearns.
Turn key off, then start it

You can try blowing out the tumbler with computer cleaner (air in a can) to remove any metal shavings that may be interfering with the Passlock being able to read the key. Also be sure thereare no other magnets (Mobil speed pass) or the such on the key ring. Beyond that it may be a computer or key issue