Don't have any ex2300 but have seen the same fan failure and then clearing within a minute or so error on MX104 devices. Since the events happen and clear so fast I just assume it is a false alarm. But have not bothered to raise a ticket on it. Just assume it is a software bug.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home
If the logs have only the SNMP TRAP for "Fan/Blower Failed" and "OK" status appearing randomly and not accompanying any alarms or other messages, they should be harmless. If there is a real hardware issue with the fan, there should be an active alarm on the chassis under "show chassis alarms" and show log messages will also have the alarm details like Major/Minor/set/clear.
Also if the fans are set to high speed due to some hardware or environmental issue, it will be reported in logs and "show chassis environment".
The below log indicates that there was some read failure for FAN in slot 0 which may have triggered the chassisd to generate the Failure TRAP. The chassisd process is responsible for monitoring and managing the hardware present in the physical router chassis, including: ASICs, power supplies, fans, temperature sensors, and managing hot-swap events.
However, if the messages are frequent, there could be issue with the hardware.
lcmd: sazmac_fan_periodics:554: FAN(slot = 0): Failed to read Status (fruid = 0, fanid = 0, status = -1)
When such logs appear please check the real status of the hardware from the CLI. If everything is fine with hardware and still logs persist, you may check with JTAC if the messages are expected in the code you are running. It seems a transient issue to me.