I have been having this crank no start problem intermittently with my 2000 Chevy Monte Carlo SS 3.8 V6. No code came up until she would not start at all. My mechanic found U1016. So I had the PCM and BCM swapped out, programmed and was told it was ready to be picked up. I drove it and felt like my girl was back. Got a little ahead of myself. I parked it and left it for a hour and boom just like before she was cranking and not starting. She finally started and I got her home. Monday I went back to my mechanic and he found P1626 and P1631. P1626: Anti-Theft System Fuel Enable Circuit and P1631: Theft System - Password Incorrect. I'm growing frustrated because I know there's nothing wrong with her mechanically. But my mechanic charged me $550 for the work he done. Earlier in the year I took her to the dealer and they said it the wire harness to the Ignition Module. That was $724. I have read that perhaps I need to change to harness for the ignition switch. Is that true? I know the Passlock system is faulty. Greatly appreciate any help you all can give me. I love my MC. My Silver Fox.

 

I don't know for sure if this will help but I purchased some repair manuals recently and although I haven't had time to check them out and read them it might be helpful understanding some of the problems.
Also you may want to google the issue and see what comes up and check that as well.
Recently a friend of mine was telling me a story similar to yours and some how a wire had not been fastened correctly and was laying on the exhaust pipe somewhere and caused the no start issue?

2000 Chevy Impala Monte Carlo Service Shop Manual 2 Vol Set Preliminary | eBay

 

Thanks for your input. I'll definitely check out the repair manual. What's crazy is that the issue intermittent. Since I got a new PCM/BCM swapped in, I'm thinking the guy who programmed them did not input the right software since the P1631 code say Anti-Theft Password Incorrect. My mechanic keeps reaching out to him, but the cat keeps ducking and dodging. Smh.

 

I am having the exact same issue and codes, were you able to trouble shoot the issue? I replaced the BCM and had dealer flash it and it's doing the same thing.