I had a rough landing with my P3P and the GPS connector on the main board broke the solder joints.  everything else works fine, and if I hold the connector down, the GPS signal is good.  I could fly without GPS.  I decided to replace the main board instead of repairing the old one.  I received a new main board (in a DJI sealed box).  Install went off without a hitch.  I have over 30 years experience working on avionics. When I fired the drone up, I got the normal lights and sounds.  It required an upgrade so I upgraded to the latest firmware.  After upgrading I calibrated the compass, IMU, and gimbal.  Everything appears to working great and the video signal is good.  When I try to start the drone I get "Invalid Serial Number".  I assume this is the Flight Controller serial number.  I checked, and the FC serial number in the DJI GO App is in fact the new serial number listed on the box.  I contacted DJI and asked if the FC serial number had been activated.  They told me no and to re-sync the RC.  I did and still the same.  I have uninstalled the DJI Go App, created new accounts, downgraded and upgraded the RC.  I even replaced all the cables.  This one has me puzzled.  DJI says the FC has not been activated, but I can't figure out how to activate.  I am starting the repair of the original board and have also ordered a Mavic Air 2 (Christmas present to me from me). I hope to use the P3P as a learning machine.  It has been good to me and has traveled around the world. Thanks for any advice.

Hi there, we are sorry for the issue. As the issue persists, after trying to update the firmware, relink the drone and RC, and reinstall the app, we may suggest that you send the drone back for further assistance.
If service needed, please start a case at https://repair.dji.com/repair/index and send the drone back to our service center, thank you.