I'm sorry I'm sure this information is on here already but I can't find anything by searching 'o2 sensor test'
I'm looking for Legacy specific instructions on how to test the 02 sensor the front 02 sensor.
I've read on the ehow advice you can unplug the harness and put a positive lead on the 'backprobe' (what is this on the Subaru, which color wire goes to the 'backprobe') and ground out the volt meter then take voltage readings.
Then you can create a vacuum leak to create a lean condition which will identify if your o2 sensor can accurately detect the lean condition and it should read lower voltages at this point.
So where is a good hose to pull to create this vacuum leak on a 05 legacy.
Also does 2.5i use the same before and after sensor as a GT?
I'm trying to do the first diagnostic test to identify my problem on the infamous p0420.