Degraded search Index partition in SharePoint

Error: "Search has encountered a problem that prevents results from being returned. If the issue persists, please contact your administrator."

For further investigation please run the below command using PowerShell as Administrator.

$ssa = Get-SPServiceApplication Get-SPEnterpriseSearchStatus -SearchApplication $ssa -Detailed -Text

After execution, the result confirmed a problem with the Index Partition, showing the Index component with the Degraded state.

Collect Logs:

get-sploglevel | where {$_.Area -like "*Search*"} | Set-SPLogLevel -TraceSeverity VerboseEx

Run Clear-SPLoglevel on your search farm and recreate the same issue and then gather the latest 2 ULS logs from each server in the search topology from the Search farm and then review the logs.

Here are plans to fix the issue: 

Plan-1:

Reboot the index server(03). If the components are not up then look for Plan-2

Plan-2:

Restart SharePoint Search Host Controller Service in all the server in search farm – this should fix the issue in most of the cases.

Plan -3:

Clear SharePoint cache in all the SharePoint search servers in the farm and then restart search services.

Plan-4:

Resent index and start full crawl.

If the index component issue did not fixed OR If the Index component keeps getting degraded and after every 10-15 minutes or so and then it recovers then proceed with the last plan.

Plan 5: If the index component status is intermittent failure then this will interfere with the sync process on other index server(04).

• Login to the degraded status index server 03 server
• Stop SharePoint Search Host Controller Service
• Stop timer service
• Delete these 3 folders (indexmeta, journel, ms) in two partitions under the path below. I:\indexinternal\Index0\SP1e52c35718bb.1.I.1.0 I:\indexinternal\Index1\SP1e52c35718bb.1.I.1.0

• Login to 04 server
• Stop host controller
• Stop timer service
• Delete these 3 folders (index meta, journal, ms) under the path below.
•       I:\index\Index0\SP1e52c35718bb.1.I.1.0
•       I:\index\Index1\SP1e52c35718bb.1.I.1.0
• Start search host controller and timer service on both 03 &04
• the 3 folders are provision on the path above on Index 0 & 1
• we can see the folders then we go to UI and do the Index Reset
• Start the full crawl on a content source and we can see success crawl log 
• All the search components are should in healthy status.