2002 Chevy Impala Will Not Crank, Will not communicate

I was working on an overheating problem with this vehicle and among other things changed out the upper and lower radiator hoses. The upper hose change required me to move the air box (with the ECM in it) to one side. This may or may not be related, but now it will not crank. I did some research, used a 2.2k resistor across the yellow and black passlock sensor wires going into the BCM, did the relearn process, got the security message to go away, and it still does not crank. After another battery disconnect/reconnect the security message has returned. What's more, it does not talk to my Innova 3140 code reader. I have confirmed continuity from the PCM to the DLC on the serial data line and the associated ground line. I have tried a different PCM (used). I am also getting a "service vehicle soon" message, and out of curious frustration disconnected the PCM from the vehicle and turned on the key. The results are identical, it's as if the PCM isn't there. I have confirmed 12VDC at the battery voltage pin of the PCM, and checked continuity to my ground points. The related fuses all look good. The big puzzle is why it won't crank, but I also don't understand why my reader can't talk to the PCM. I have a fairly strong electronics background, and am being tempted right now to go more into computers simply so this sort of thing doesn't floor me again! Any assistance would be appreciated