there is one on the muffler side of the CAT.

I guess I will do change that once I change my fuel filter.

Don't change it yet! Corvettecrazy asked a question I was going to ask (I think mine'* getting lazy). I've heard that one is more important than the other. I think one of them can be left original for a long time.

Gearheads?

You only have one Steve, Don't worry about it

Usually the front ones on OBD II cars are the ones you need to replace, the rears are there to keep an eye on the Catalytic converter. The front one senses the temp of the exhaust gas, and the second one shows the PCM what changed.

Is it safe to say that the rear can remain original, then?

Also, does anyone have the GM part # for the front O2 on a 99 L36?

I don't know recommended change intervals, but I don't know if i would let the rear one go past 100k. From my understanding it really is there to trip a SES light for the cats, since OBD II is heavily scrutenized during emissions testing, etc.

I believe the front one is the one that controls Gas Milage.

Also, does anyone have the GM part # for the front O2 on a 99 L36?

http://www.bonnevilleclub.com/techinfo/?article=49

Isn't this it?

I thought the K was the L36

It is..you're right. I'm going to order a new one right now... 98K and still on the original. Gas mileage is slipping... Down to 19 MPG.