Subaru Problems? Ask a Mechanic for Answers ASAP
Your P0558 is brake booster pressure sensor.
The cause is you have fitted the wrong alternator.
Your alternator should have 3 pins, the 3rd is for "smart charge".
Some aftermarket alternators may have the 3rd pin, but does nothing.
Go back to supplier.
I think you'll find your non-genuine alternator is the issue.
This code comes up when you fit a non-compatible alternator.
What was wrong with the original, can you put it back on to see if the code goes away?
Or have you asked Subaru, because they will tell you the same thing?
What is the price difference you're dealing with here?