I'm reading codes 12, 51, and obviously 55 (ignore)

but I just had the fuel system replaced and now its driving worse. I can floor the gas pedal and get up to about 20 mph. and its just a shaky ride it seems to accelerate when it wants to.

I looked up 12 as a battery thing
I also looked up 55 has a fuel system lean,

what can I do to fix this?

(Mind you i've had shady mechanics in the past few months get a hold of my car.. now i'm trying to correct my dads mistake. ;-( )

 

What was replaced in the fuel system?

 

Probably a bad upstream O2 sensor. Check the wiring first to make sure its connectod good and free of dirt.