Hi Guys!
My son’s 2001 GMC Sierra p/u is being a stinker! From time to time the truck refuses to start. I noticed that the security light comes on along with powering up the rest of the vehicle, but the engine will not turn over. My son who lacks any patience has been taken to the cleaners having the electrical system to the alternator replaced. Still the problem exists. Old mom here believes that it is the security system that for whatever reason is kicking in and being fussy. Last time he was home on leave, this event occurred. I was able to get the keys away from him and go out and fiddle with the truck. After trying to start it in park, in neutral, brake on brake off, I tried then by shifting the position of the steering wheel. It started then but still not sure that this was the catalyst. So what do you think? BTW… the ignition key is standard issue… no chip. So what do you think? If it is a systemic problem with the security system? Can we disable it? Thanks! Hugs & Kisses! 
Since I have been burned before giving advice on the wrong seceuity system in order for me to help you you must find out for sure either what version of a GM security system you have or what aftermarket system you have. The thing that steers me away from a GM system is that Passlock systems (the majority or) do not disable the crank system, they allow the car to crank,start, then die. Passkey systems (with a chip in key) disable crank. Now I don’t say it is impossible to have a starter disable Passlock system so you will have to look on the sticker on the inside of the glove box door (after contacting a GM parts man and asking what code to look for) and find out what system you have.
Just as likely your car suffers from acid damage from a previous leaking Delco battery or a normal fault that is inhibiting cranking. Any Passlock or Passkey or aftermarket system can be disabed.
Blessing for the quick response! I will indeed do the research to identify the correct security system. Thanks!
I did read the owners manual and it spoke of the Passlock system, but unlike what you describe, the vehicle would NOT turn over. In fact all the other bells and whistles power up, but when turning the key, you don’t even get so much as a click… or clack!!! So, as suggested, I will find the codes, check with the G-man and write more later!!!
Interestingly enough, I read a post on a similar problem where it was suggested leaving the key in the ignition for 10 - 30 minutes to allow the chip to reset itself. Being that I still don’t think there is a chip there, I tried this measure as a last resort… and of course prayed for Divine intervention!!! After the 30 minutes had passed, my son went out and the truck turned over for him!!! Can I hear an AMEN from the chior please!!! 
So for now, he’s up and running. Needless to say I will follow through with your advice as I fear this odd ball event will rear its ulgy head in the future. It would be nice to have a clear understanding of why and how to fix it if it should reoccur. Thanks for all you do! I’ll be back to bother you once I have more information!!!
Best Wishes Always! :0)