I just pulled a code of 0510, which say max voltage on downstream O2 sensor. I reset it, and so far it has not come back after letting the car run for 10 mins or so. But I want to see where it is in case I want to change it later... My Haynes manual is flat wrong, it says the downstream sensor is in the cat converter under the middle of the car near the resonator - not in my car (99 SE 5sp). It looks to me that I can see a cat converter right under the engine, looks like both exhaust manifolds dump into it. I see an O2 sensor just below the manifold to Y-pipe connection on each bank, I assume those are the 2 upstream ones. But I also see another one sticking out of the front of what I guess? is the front cat - is that the 'downstream' sensor?

 

Downstream means behind a catalytic convertor. Federal spec comes with only one and it's behind the main cat on the b pipe. Cali spec will have 2 - both on the y pipe after the pre cats.

 

the upstream ones are at both ends of the y-pipe, downstream is fight after the cat

 

Believe me, there is no O2 sensor anywhere on that cat in the B-pipe just before the resonator (I looked and looked). I count 3 of them up near the engine, one on each side just below the exhaust manifold to Y-pipe connection (upstream I guess), then a third one sticking out of what I guess is a pre-cat as you call it. My car is from NY, so maybe it has California type emissions (not sure about that though). So far the code has not come back, but if it does, I just wanted to be sure of what I was gonna replace... Thanks for the info, but does anyone else have a 99 (mine was built in 9/98) that does not have an O2 sensor sticking out of the B-pipe cat? Surely there is a reason it is not there as it was on earlier years...