2002 mustang won’t start theft ligh stays on

I lost keys to my 2002 mustang v6 so I went to salvage yard and pickup lock cylinder with key and Ecu from a mustang that was the same year and same engine and it still won’t start I program the keyfobe the doors unlock and lock trunk opens from remote any one know what’s going on Thanks .
 
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You forgot about the CLUSTER.

The cluster and PCM are programmed to each other. Changing either one or the other will invalidate PATS. Just as with loosing the keys.

My recommendation. Put the original PCM back in because it's the one that matches your car's VIN. The look over this information about a possible DIY option to reprogram new PATS keys.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Or call a mobile lock smith to re-program the new keys that you have. Regardless I recommend putting the original PCM back in.
 
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