When monitoring Linux hosts in Nagios XI, the following error was being reported for the CPU Stats service:
"UNKNOWN: iostat not found or is not executable by the nagios user."
This is due to the check command having a dependency on the performance and monitoring tool sysstats, in order to resolve the issue install the package as below:
apt-get install sysstat
On the next service check you should receive a valid service status and information message.
Deans Blog 