Issue

VMware ESXi vSphere client host Health Status (Configuration tab) shows Unknown and Question marks (?) next to each component.

Trying to Reset Sensors fails with error:

Error
A general system error occurred: Reset System Health Failed for One or More Sensors
Call “HostHealthStatusSystem.ResetSystemHealthInfo” for object “healthStatusSystem” on ESXi <server> failed.”

Resolution

Issue was resolved by restarting CIM Server.

• Go to Configuration tab > Security Profile (in Software section)
• Click on Properties (next to Firewall section) 
• Select CIM Server and click Options
• and then Restart

DELL PowerEdge 2950
VMware ESXi 5.0