Mine has 2 they were listed for an 02 and were the same part number

 

My truck is throwing the same code, P0138.

Which sensor was it for you guys after all? Pre-cat? and what part #/brand? Thanks

 

Bank 1 Sensor 1 = Pre-Cat
Bank 1 Sensor 2 = Post-Cat

Bank 2 Sensor 1 = Pre-Cat
Bank 2 Sensor 2 = Post-Cat

If you have a sensor 2 code, that means one of two things. The first, and most likely, is your catalytic converter is going bad; the second possibility is that the O2 sensor itself has gone bad.

Walk into any parts store, I prefer Advance Auto or O'Reillys, and tell them your model year/make/etc., and tell them you need the downstream, or post-cat, O2 sensor for it. Depending on brand name and model year, it may be a bit of trial and error. Just keep the receipt handy so that if the one you get doesn't fit properly, you can take it back and exchange it for a different part # in their system.

For example, when replacing my pre-cat O2 sensor a while back, I was handed Bosch part # 13122. The wiring on this sensor, however, was not long enough and could not reach the plug it was supposed to connect to. I went and exchanged it for a different NGK sensor, and that fit perfectly. For 2001s (and I'm assuming 1999 and 2000, maybe even 1998), the correct Bosch part # is 13100 for pre-cat, part # 13280 for post-cat.