86 Prelude o2 sensor
 

Love reading the ECU in these things- so easy to find and figure out.
Anyway, after the PGM-FI light comes on my ecu tells me it's a bad
master O2 sensor. I bought one although the Autozone fellow said two
were recommended. I was lookin' around in my Haynes manual and can't
find where the second one IS. Anybody know? I'm thinking the master is
the one right up front sticking out of the exhaust manifold shroud
(which I just tried to take off to give me easier access to the sensor-
rusted bolts made that a screaming nightmare- gotta go buy myself a
deep socket wrench), is that the master? Is it really necessary to
replace them both at the same time?

Thanks,
Bill