Bcm and ecm question
 

If you have a faulty bcm and pull one from a yard matching your vehicle, you need to have it flashed by a dealer to get it to work. Instead of that why couldn't you pull both the matching ecm and bcm out of the same vehicle instead? Why would wouldn't it work and save yourself the money and grief of reflashing? Granted you would have to do a relearn,ok, but if you bypassed the key(resistor, etc.) why shouldn't it work? Thanks.

depends on the vehicle build..some have a separate theft module outside of the bcm and ecm.

if you have a transponder key you couldn't relearn the key to the new pcm would have to cut and program a new key.

I think you would need to replace EVERY module, and modern vehicles have quite a few.