Do a Full Auto Scan for all other modules and see if you have any fault code in any other module...........Specially in Instrument Cluster and BCM........

What was the reason of changing the old ECU.............? And after you have programmed used ECU dose the vehicle engine crank properly............?

Could you please collect a datalog of this problem and upload to us? Once uploaded, please send email to support@autel.com.

 Have you checked the basic things.............? Like............Fuel, Spark. Cylinder Compression, Valve Timing..............?

 Dat'szadoozy. See if there is something under  special functions called Parameter Reset and give that a try. U -codes are usually  communication related error codes, and Parameter Reset gives the modules on the Network sort of a B@tch slap to get them to shake hands and communicate between each other. Mazda has been in bed with Ford for years and share technology between each other

 Vehicle does not start anyway, so I would think, What do I have to lose by trying? But before I try anything  I would just make sure with a Mazda dealer that by VIN # tear tag, or Calibration #, that the used PCM I obtained is the correct part # before I waste any more time.I would also make sure the good  battery has been fully trickle charged overnight and maybe have cables attached to it from a battery charger or off a idling  car to keep battery  from going flat.

  If you have the erase-keys ability, you should have the programming as well.