Oracle Enterprise Manager 11g Database Control - Agent Unreachable
Do you have separate Oracle Homes for your Database and Grid Infrastructure (which was new to 11gR2, IIRC)?
If so, then the Agent probably runs from the Grid Infrastructure's Oracle Home.
Annoyingly, the bin directory in both Oracle Homes contains a program called emctl but only one of them will work as you expect!
If in doubt use the Linux find command to locate anything called emctl ...
find ???  -type f  -name emctl  -ls 

I'd suggest, once you've found it, you add the Agent's &quot;Oracle Home&quot; into /etc/oratab under its own name, to make it easier to find it next time.