ORA-16766 Error in Oracle Data Guard: Causes and Fix (Oracle 19c Guide)
Full Error: ORA-16766: Redo Apply is stopped
This error appears in DGMGRL SHOW CONFIGURATION output against the standby database. It means the Managed Recovery Process (MRP) on the standby has stopped and redo is no longer being applied. The standby is diverging from the primary with every passing second.
It was 3:22 AM. The monitoring alert fired: "Data Guard configuration warning — ORA-16766 on standby." Apply lag had jumped from zero to 47 minutes in under an hour. The standby database was alive, connected, receiving redo, but not applying any of it.
ORA-16766 is one of the most common Oracle Data Guard errors in production Oracle 19c environments. It always means the same thing: the MRP process on the standby has stopped. But the reasons it stops, and the correct fix for each reason, are completely different.
This guide covers every root cause of ORA-16766 in Oracle 19c, the exact DGMGRL and SQL commands to diagnose it, and the step-by-step fix commands for each scenario. Most ORA-16766 errors are resolved in under 5 minutes once you know which cause you are dealing with.