The cores, logs, and work folder, plus current log file, are located here:
/var/lib/fahclient
The config.xml file is located here:
/etc/fahclient
The FAHClient is started and stopped via the service script in /etc/init.d/FAHClient.
sudo /etc/init.d/FAHClient start
sudo /etc/init.d/FAHClient stop
Both stop and start may be combined via this command:
sudo /etc/init.d/FAHClient restart