Occasionally you may read a vehicle only to be greeted with a checksum warning. The warning says you may have a permanent DTC or the car may not be drivable.
In 90% of scenarios checksum errors are harmless. Some of the possible causes of a checksum error are listed below:
You have an early BA with a blank VID block from the factory
Someone used Ford IDS improperly and wiped the VID block
You have a vehicle which was tuned with an early CAPA flash