May be you should make a diagnostic and you can do it in a garage but if you want to do it by yourself you could buy a diagnostic tool here: spobd2.com
SOURCE: 99 GMC Sierra Pickup: Crank but no start. starts
The theft light issue is a good observation. Have you replaced the key tumbler or an ignition component lately? Has there been a lightning storm close very recently?
What the flashing security light is indicating is an attempt to relearn the key you are using.
Turn the key to run position, allow light to flash for 10 minutes, when light stops flashing turn ignition to OFF position for 5 seconds, turn ignition to run position and start over. Do this for 3 consecutive cycles, the the theft system will relearn the key cylinder code and store it once again.
Try that first before you go any further.
Let me know what you find.
Regards,
SOURCE: 2000 GMC Jimmy won't start
i had the same problem and replaced the fuel pressure regulator. Cost about $45 at advance auto store or about $90 at napa auto stores. Once i replace that my truck started right yup and runs like a champ
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