Error 121 is a generic “failed to program” message, so this can have several causes.
One common cause is low battery Voltage to the ECU that is being programmed.
Check all connections, and ensure that the vehicles batteries are good and
fully charged. You can also remove power from the ECU that is being programmed, wait,
connect, and try a second time.
If you still get error 121, remove power for a short time, try again with a different adapter.
Vocom , NexIQ, Noregon DLA or Dearborn.