I just received some O2 Sensors for my '99 XLT (4x4, if that makes a Dif.?) 4.0, and am wondering: Which side does my Upstream go on? I'm told (via Google) that it goes "Before the Catalytic Converter" but when they say that, are they referring to the Side faces the Block?
I know I sound stupid, but when I see stuff like this, I can't help but think of how they call the "Back" of a Semi Trailer Near the Doors, and the "Front", near the Cab. Something that has always perplexed me... So, before I crawl under this thing and working on it, I'd like to be in the Know.
Thank you for helping this Noob get a grip! ('Preciate y'all!)

 

Upstream would be block side.

Think of the flow of the exhaust. It starts at the manifold, flows downstream to the cats, then the muffler, and out the tail pipe.

And the semi trailer the front is near the cab since it is closest to the vehicle moving the trailer. It becomes one with the vehicle.

 

Right on Vista. THANK YOU for sharing those Great descriptions in a way that I can 'get'!