changing both O2 sensors

[merc81]

After I put the rebuilt transmission back in and put everything back together, I discovered the upstream O2 sensor did not work. I replaced
both upstream and downstream sensors while I had the engine out, so I thought I could remove the one on the engine without much trouble--



That's the sensor hiding up there behind the exhaust flange on the manifold.
I removed the two nuts holding the exhaust pipe on the manifold, and the three bolts holding the exhaust pipe to the underframe:


This didn't give me much room, but I was able to get my arm up in the hole the exhaust pipe left and not quite-maybe get a wrench on the O2 sensor. Turns out I had to use a line wrench rather than a normal open end wrench (7/8 did it) to just barely grab the end of a face on the O2 nut. If you are changing out one that's been in there for a while, I'd cut the wire from the sensor and put a deep well socket from a large drive set over the sensor. They are pretty tight up there.
I was lucky because I'd just put this sensor in two days before while the engine was removed--it just was defective (no comments as to where I bought it, I'd suggest you get one locally from NAPA ).
Here is a top view from an engine that's open:

I'd say this is a 1 hour job unless you have trouble breaking the sensors loose.
Here is the downstream sensor