Recently I came upon a default self-signed PXE certificate expiration on a ConfigMgr PXE service point site system. The warning message is:
Warning: The certificate associated with this media will expire in hour(s) and minute(s). A valid certificate is required for the duration of a boot media based deployment. Do you wish to continue?
To re-up the self signed certificate is quite simple, but a few extra things need to be done as well once the certificate has a new expiration date. First we navigate in the console to the server which clients are PXE booting from under site systems and look at the properties of the ConfigMgr PXE service point role. 
Click on the database tab then set the expiration date out. Now we have to go to the Boot Images node in the SCCM console and update the distribution points (follow the wizard) for each of the boot images that contain the expiring certificate. Then restart the WDS service on the server and you’re all set.
elgwhoppo's vNotebook 