No crank no start Problems solved!!!!
I hope that this read can be found by so many others with these same and posted issues.
First, the engine started acting up , backfiring no power and erratic rpm reading on dash, and Engine stalling at any speed. Skipping the basics like poor battery condition and loose cables, Others had posts to check replace crank sensor then wiring or faulty ECM.

I purchased and replaced the crank sensor with a "Standard" brand part from a local store. I did 2 laps around the block and the car died completely. This was the beginning of a 3 month process but I refused to give it up to the dealer. I assumed that the new crank sensor was not a problem THIS WAS MY FIRST MISTAKE.

I spent $900 on a refurbished ECM and did key relearns. I didn't know that changing an ECM will put the car into lock down.
This is what caused the "No crank" issue and all those funky described dash light issues and errors, loads of DTC's. I have seen so many C6 owners post of these issue but I never had seen a report of correction.
I did significant wiring and voltage tests. Spent hours with a friends Snap on scan tool looking and clearing DTC's, checking functions. Only one stored DTC was correct P1631.

Then I found something on a Cadillac forum:DTC P1631 Theft Deterrent Start Enable Signal Not Correct

"The remote control door lock receiver (RCDLR) sends the fuel enable password to the engine control module (ECM) via the serial data circuit. The ECM verifies the fuel enable password received matches the password stored in memory. If the fuel continue password is correct, the ECM enables the starting and fuel delivery systems. When replacing a ECM with a GM SPO replacement part, the new ECM will learn the incoming fuel enable password immediately upon request."

You can do this on your own, IF you don't have a vehicle configured for export out of the US:

Important: This procedure is not available on vehicles equipped with EXPORT configured modules.

1. Press and hold the OFF/Acc button for approximately 4 seconds to turn the ignition ON, with the engine OFF.
2. Observe the Security indicator. After approximately 10 minutes the indicator will turn off.
3. Turn OFF the ignition, and wait 5 seconds.
4. Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.

Important: The vehicle learns the password on the transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.

Start the vehicle. The module being programmed has now learned the RCDLR password.
With a scan tool, clear any DTCs.
After I did this procedure, the engine cranked over again. I put the old OEM crank sensor in to test, and it immediately started. Put new "faulty" sensor in again and no start.

Replaced the crank sensor again but with a "new AC Delco" brand, cleared DTC's and no problems ever sense.

Long read but these posts thread through the internet and hope that it may help somebody else

Thanks for all you input

 

After you installed a new AC Delco CPS, did you have to do a CPS relearn with a scan tool or did you just start and go? The reason that I ask is that in another thread I was told after a CPS replacement, a CPS relearn procedure with the appropriate scan tool had to be performed because the ECM won't relearn the CPS own its own.

 

Nice job sticking with it until you found the problem and fixed it.

And big thanks for posting the fix - too many times these threads just end with no conclusion.

 

Even though the car runs great again , I think I need to do the re-learn.
I have the Snap On scanner so i will report back

 

Any part with “sensor” in its name should ALWAYS be replaced with OEM !!!

 

I scanned again and there are no DTC's.

I did not do a crank re-learn after replacing the CPS

So far so good and Im about 1000 miles down the road now.
Im starting to trust the car again.