4194：indicates that undo and redo are inconsistent（Arg [a] Maximum Undo record number in Undo block，Arg [b] Undo record number from Redo block）
How to Fix:

1. Log in and start mount status.
[oracle@oracle ~]$ sqlplus/as sysdba
SQL*Plus: Release 11.2.0.1.0 Production on Thu Aug 1 16:08:26 2019
Copyright (c) 1982, 2009, Oracle.  All rights reserved.
Connected to an idle instance.
SQL>  startup mount;
ORACLE instance started.
Total System Global Area 1.3429E+10 bytes
Fixed Size                  2217992 bytes
Variable Size            1.0335E+10 bytes
Database Buffers         3087007744 bytes
Redo Buffers                4661248 bytes
Database mounted.

2、View undo parameters
SQL> show parameter undo;
NAME                                 TYPE        VALUE
———————————— ———– ——————————
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1
SQL>  select name from v$datafile;
NAME
——————————————————————————–
/u01/app/oradata/orcl/system01. dbf
/u01/app/oradata/orcl/sysaux01.dbf
/u01/app/oradata/orcl/undotbs01. dbf
/u01/app/oradata/orcl/users01. dbf
/u01/app/oradata/orcl/example01. dbf
/u01/app/oradata/orcl/IBIS_DATA_TS. dbf1
/u01/app/oradata/orcl/IBIS_DATA_TS. log1
/u01/app/oradata/orcl/DATAM_TS11. dbf
/u01/app/oradata/orcl/CQDX.dbf
9 rows selected.

3. Set to manual and restart
SQL> alter system set undo_management=manual scope=spfile;
System altered.
SQL> shutdown immediate;
ORA-01109: database not open

Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.
Total System Global Area 1.3429E+10 bytes
Fixed Size                  2217992 bytes
Variable Size            1.0335E+10 bytes
Database Buffers         3087007744 bytes
Redo Buffers                4661248 bytes
Database mounted.
Database opened.

4. create a new tablespace, modify the parameters to use the new undo tablespace, and reboot.
SQL> CREATE UNDO TABLESPACE UNDOTBS02 DATAFILE ‘/u01/app/oradata/orcl/undotbs02. dbf’ SIZE 3G AUTOEXTEND ON NEXT   10M;
Tablespace created.
SQL> alter system set undo_tablespace=”UNDOTBS02″ scope=spfile;
System altered.
SQL> alter system set undo_management=auto scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.
Total System Global Area 1.3429E+10 bytes
Fixed Size                  2217992 bytes
Variable Size            1.0335E+10 bytes
Database Buffers         3087007744 bytes
Redo Buffers                4661248 bytes
Database mounted.
Database opened.
SQL> show parameter undo;
NAME                                 TYPE        VALUE
———————————— ———– ——————————
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS02

Done!