12/55 diag code is normal.

if there's no fuel flow either the electrical circuit is open (blown fuse
possibly) or the pump is bad or the fuel line is blocked (mashed or
pinched). If you have voltage and ground going to the pump and if the tank
is near empty it's not too difficult a task to drop the tank and replace
the pump & pickup filter. If I went to the trouble of dropping the tank, I
would replace the pump and pickup filter no matter what condition the
original was found to be in (whether it fixed the problem or not).

Just because you hear it doesn't mean that it is working.  A bad regulator,
leak in the pickup, leak in the output, or a number of other problems could
let you hear it run while it actually isn't pumping anything.   Like another
poster suggested, I would replace the pump assembly, it is about due anyway
and work from there.  If you hear it, then it is getting power so that is
not your problem and unless the line is completely crimped shut, you should
still get pressure when the engine is not running so I doubt that it is
either of those problems.  Did you have it pressure checked?