C0042 SUBARU - Power Supply Voltage Failure
C0057 SUBARU - ECM Control System

Check all engine grounds, make sure battery connections are clean and tight.

Your engine may be slightly different but these images may help:

Ground locations 2004 Outback Wagon

I'm used to working on Hondas and Toyotas which have a body ground strap that tends to rust out over time. Does the 2004 OBW 2.5L have a body ground? Where does the main fuse box ground? Thanks!

www.subaruoutback.org

 

I agree with Silveronyx, check battery and ground cables.

C codes are communication codes which rely on good clean wiring connections and a good battery.

Subaru ECM’s rarely fail and this is unlikely to be your problem.

Seagrass