ORA-12012: error on auto execute of job [num]
If this is a job created with the dbms_job package then you can find information using the following select
select * from dba_jobs where job=[num];
If you have created the job using the dbms_scheduler then execute the following:
select obj# , object_name
from sys.scheduler$_job a, dba_objects b
where a.obj# = b.object_id
The using the EM Dbconsole you can find more details about the job.