Cisco - Firewall - Cisco ASA 5505 - Restoring Cisco ASA 5505 to Factory Defaults
Go into enable mode:
en
Configure terminal:
config t
Set to factory default:
config factory-default
Hit the space bar at each of the “more” prompts.
Now write to memory:
wr
Confirm with:
show start
If you see an error:
ERROR: command can only be executed in single router mode
Erase the startup config:
wr erase
Then reload (Don't save current config):
reload