My mom got a check engine light in her Outback. Code reader told her that it was an O2 sensor on the second bank. Which side is that?
The only side. 
Its the cat efficiency sensor, the second one in the stream.
Ah, I thought it was either the left or right bank. Guess it’s neither. Universal OK, or should I get an OE replacement?
You’ll save money with a universal, but have to do some soldering. It will be okay for that sensor, it doesn’t do much of anything except make sure the cat is doing its job.
I’m assuming its a 4cyl Outback, too…I have no idea how the H6s are set up.
If it is a 4 cyl which it probly is, regardless of year the second O2 only has one bank, in 04 subaru switched to two long pipes with 2 separate O2’s in the front. They are the same part number for both fronts though. Just find out which one is throwing the code. What code is it throwing? is it a 420 code? cats are relatively common too.
It’s a 4 cylinder Outback, 2004 I believe, I’ll get the exact code tonight. But I was told it read O2 sensor - second bank.
943kgt beat me to kit, i would have said the same. and not just because i work at the dealer (as does he), go OEM, dont pay for the part twice. I hear the sad tale too often “i bought an 02 aftermarket and it still didnt fix it…”