Hello I iwll help you with your question,
It seems the heater on your forward air/fuel ratio sensor has failed or the wiring to the heater is shorted to battery voltage. It is likely that the fault is in the sensor so you will want to get a replacement from Subaru for part number 22641AA211
The sensor is located at the front of the catalytic converter as shown in the diagram below
I will post the diagram in the next post
Let me know what questions you may have about this
Sorry I grabbed the wrong diagram, here is the correct one.
The first diagram I posted (since deleted was for the Forrester XT and that is why I had to post the correct diagram in the next post. Sorry if there was any confusion but that is the correct diagram for your vehicle.
The auto and manual uses the same exhaust so the diagram still applies to your vehicle.
Yes the Denso sensor will work fine for your vehicle, remember no grease on the connector!
Glad to help and thank you for using JA!!