When ESXI 5.1 in vSphere is launched,  you click on the configuration tab and click the system health, you will get all gray question marks on the items and when you click the reset sensors you get the below error.

Call “HostHealthStatusSystem.ResetSystemHealthInfo” for object “healthStatusSystem” on ESXi “esxi_srv01” failed.

To solve this issue you must go on the Configuration, click on the Security profile, click on the Firewall and click on Properties.

Find the CIM-SERVER entry, click on the Options and Restart the service.

Close the vSphere and re-open it. Try to reset the Sensors.