Joth

I've been searching through the threads about this sensor, but I cant seem to find a part number for this. In my MY98 Forester i've got the brown plug sensor thats mounted to the front of the coolant cross pipe, I got a CEL code 21 yesterday after having noticed the high idle cold starting for about a week, can somebody tell me what the part number is for this sensor and what year models used the same part?