■ [ERROR_SYSTEM_DOWN] is detected when system CPU monitoring daemon (wmond) is downed.

This does not mean system down. Jennifer classifies as an error in case of no response of data from wmond for more than 8 sec.

In spite of normal wmond deamon operation, error can be detected scarcely.
In this situation, you should consider that wmond does not execute because of 100% of CPU usage of the system, or think about UDP data loss due to N/W issue
or think about UDP data loss due to N/W issue

■ [ERROR_JVM_DOWN] detected when “The Java Process” is downed.

Each WAS agent sends starting up information to Jennifer Server based on default value 1sec.
If such information does not arrive within time of agent_death_detection_time = 8000(8sec) which was designated on Jennifer elements settings, Jennifer Server classifies the agent is downed.
As an exceptional case, ERROR_JVM_DOWN might occurs when response time delayed (more than 8 sec) under the process of Full GC, or wmond does not execute because of 100% of CPU usage of the system
And, ERROR_JVM_DOWN also may occurs in case of hang up situation (More than 8 sec) due to dumping javacore or heap dump by executing kill -3 on Java process.