The P0028 code is set when the pressure sensor switch condition (closed, open) does not correspond with the duty cycle signal that's being sent to the related solenoid.
The solenoid controls oil pressure to the variable valve lift cams while the pressure sensor checks to see if the pressure is, in fact, high when it should be high and low when it should be low.
The troubleshooting tree starts out by checking the wiring between the engine control module and the pressure sensor switch, rather the solenoid. Presumably the idea is to make sure that the pressure sensor function is okay, and after that, presume that the fault must be the solenoid.
There have been cases where the pressure sensor switch failed -- in some the sensor was (visibly) leaking and wouldn't respond properly to the actual oil pressure.
The solenoid, of course, could also fail to function properly; however, if there were an open or short in the solenoid or the wiring to it, that would generate a different trouble code.
This post (My 2007 Outback is leaking/Burning Oil - Subaru Outback - Subaru Outback Forums
) has a good photo of the left bank solenoid (on a 45 degree angle) with the aqua/pale blue connector, and the pressure sensor vertical to the left of the solenoid with the white connector. (Read down for cautions when installing the switch, and to post #19 for another P0028 case caused by the pressure switch, not the solenoid.)