If you changed a transponder on a kart, you need to configure this in the software.
If you are using Fast 5:
1. Open the Fast 5 Configurator or Timing software
2. Click the menu icon and go to Activities> Transponders
3. Search for the new transponder, or add it by clicking the plus '+' symbol above the list
4. If it's not opened yet, open the details of the transponder by double clicking the item in the list
5. Click the search icon (magnifying glass)
6. Choose the kart for this transponder, and click 'Select'
7. Click 'Save' to finish
If you are using Fast 4:
1. Open the Fast Configurator
2. Expand the 'Mediums' section
3. Click on 'Vehicles'
4. Find the kart in the list and click it to open the details
5. In the details list, go to the 'Transponders' list in the bottom
6. Double click the current transponder to edit it
7. Change the transponder number to the new one you just installed
8. Click 'OK' to save the transponder
9. Click 'OK' to save the kart