I can't ping 8.8.8.8 if default interface is down

[dims]

I can't ping 8.8.8.8 from the box itself if default interface is down.

Simultaneously, I can ping 8.8.8.8 at the same time from other machines in the LAN, connected to Internet via pfSense.

Why and how to fix?

[GruensFroeschli]

Are you using 8.8.8.8 as the monitor of your default interface?
Setting an IP as monitor pins it to the interface for which it is set as monitor.

If you want to be able to use 8.8.8.8 as monitor IP from behind the pfSense, use another IP as monitor.

[dotdash]

Generally, you set the lan outbound rule to use a failover group, but the firewall itself does not. This is usually not a problem, but there is a setting under advanced, misc. to allow gateway switching.