Hi,
In that case Try doing DCPROMO /Forceremoval and make sure if any roles are there transfer to good Dc.
If DCPROMO /Forceremoval failed you can do metadata clean-up of the DC run the blow script from PDC server and then delete the entry of bad DC manually from below manual steps. Everything is done you can format that bad DC.
Script:
https://gallery.technet.microsoft.com/scriptcenter/d31f091f-2642-4ede-9f97-0e1cc4d577f3
Manual Steps
Dnsmgmt.msc [Dns Management]
A.Expand the forward lookup zones\_msdcs folder
i. Make sure only the actual domain controllers are listed, delete wrong Alias recordsremove wrong name server records
ii. Select the container [forward lookup zones\_msdcs.domain.com\dc\_sites_\sitename\_tcp] > delete incorrect _ldap and _kerberos records are listed.
iii. Select the container [forward lookup zones\_msdcs.domain.com\dc\_tcp] and delete incorrect _ldap and _kerberos records
iv. Expand the [forward lookup zones\_msdcs.domain.com\domains\guid\_tcp] and delete incorrect _ldap entries
v. Select [forward lookup zones\_msdcs.domain.com\gc] delete incorrect HostA records
vi. Expand the [forward lookup zones\_msdcs.domain.com\gc\_sites\sitename\_tcp] delete incorrect _ldap entries
vii.Select the [forward lookup zones\_msdcs.domain.com\gc\_tcp] delete incorrect _ldap entries
viii. Select the [forward lookup zones\_msdcs.domain.com\pdc\_tcp] delete incorrect _ldap entries
B.Expand the forward lookup zones\domain.com folder
i.Delete Host(A) records of dcs which are non-existant.
ii.Correct the NameServer (NS) records
iii. Follow steps similar to A ii >> A viii
Dssite.msc [Sites and Services]
A.Expand the [Sites\Sitename\Servers] delete incorrect servers
B.Delete incorrect subnet configurations [Sites\Subnets]
C.Delete incorrect site links [Sites\IP]
Make sure the domain controllers are pointing to the correct dns servers in tcp\ip settings.
Force replication repadmin /syncall