Seems the use of a fuel-air ratio sensor is specific for the MT Legacy.
I had this same code P1133 and the OBDII said however (LOW) below and I was able to clear the code by replacing the sensor. Used a Denso 234-9018. Do not buy a UNIVERSAL O2 sensor. It will NOT work!! Needs to be OEM on this car. Believe me I tried it. The OE unit made by Denso can be bought at auto parts stores and is the same as the dealer OE part. 3x check that they sell you the right unit for a MT Legacy which is not the same at the AT one.
It seems the diagnostics pathway attached above is just for a high condition.
My Legacy had 115k, started to smell a little rich, and then on cold mornings, idle RPM would be very high for a few minutes and then when the car warmed was fine. Seems that both the sensor heater was messed up as well as the actual o2 sensor part.
The sensor came out easily despite the awkard angle.
I spend hours on mind so wanted to at least share my experiences.
