Set the field to required=yes on the underlying table. You can also include a default value so that the combo always displays something. Of course, your user might put in "good" when it really is "Faulty". Another method I have used in the past is to perform an IsNull check on the field before a save, close or move record event, and on before update. Such as
If IsNull (me.cboGoodFaulty) Then
MsgBox "You must choose either Good or Faulty", vbOkCancel, "My App Name"
Cancel = True
Else:
Let the user save, move add, close etc
End If
Once the user enters the field, they cannot do anything else until valid data has been entered. Even in this case they may still lie about it being good or faulty.
I haven't failed, I have just found 10,000 ways that it won't work!