[GTP Dad]

There are a couple of issues that I would have them check. The first one is the crank position sensor. It can go bad at any time and it usually will not give you a check engine light and the dealer will insist that this couldn't possibly be the problem but trust me sudden stalling and no restart is usually the crank position sensor.

Another possibility is the security system but you said that there are no lights on. If the small red light is on then you will need to reset the passlock system. It will often disappear after about 20 minutes that is why it probably started when the dealer got the car. If it is the security system it can be reset by setting the parking brake and turning on the ignition for 10 minutes then turn the key off for 10 seconds. Do this three times and on the third time start the car. Make sure you use an OEM key and not a non-GM key.

I hope this gives you a place to start and welcome to AF!