C2500 cranks but no start, while flashing security light 3 times

[Ryanl99c2500]

Hi all, I have a 1999 gmc c2500, and it has a newer 5.7 vortec that was swapped into it with the matching transmission. Just about everything with it is new, and has been checked. It won’t start, but will crank and run for about 5 seconds or so then flash the security light 3 times. I’ve tried to re learn a key to it with no luck. Does anyone have an idea of what might be happening? I would add a video, but I don’t think I can

 

[618 Syndicate]

Security relearn will take about 30 minutes, that's your short term fix. It WILL happen again.
The only permanent fix is to get the VATS tuned out of the ECM.

 

[Keeper]

courtesy of @east302

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[618 Syndicate]

courtesy of @east302

You must be registered for see images attach

Pro tip: hook a battery charger to the truck.
YOU MUST WAIT 10 MINUTES for the light to stay out or the process will not work. Not 9 minutes and 30 seconds. This is where most people screw this up. Also the 10 seconds between cycles is important as well.

 

[HotWheelsBurban]

Security relearn will take about 30 minutes, that's your short term fix. It WILL happen again.
The only permanent fix is to get the VATS tuned out of the ECM.

Yes indeedy! Been there done that with Burb, that is the problem that led me to this forum 5 years ago. The relearn procedures would work a couple of times, then back to square one....
I had a shop recommended to me, that did a lot of tuning and hop ups to LS platform vehicles, and they said they could do it. Cost me $250 which sounds excessive but we were at our wit's end with this truck. Cause of course we could go places in it for three or four days and start and stop multiple times, with no problem, and then invariably it would act up when someone has a schedule to keep to.
Hated to spend the bux but it fixed that problem!