You are confusing between air flow meter and O2. O2 is always located by the
cat(s).Why do you think you have to replace it? What problem(s) are you
experiencing now?

Well I have 2 1989 300Es and the one my girlfriend uses is tough on cold
and sometimes warm startups and is getting really shitty fuel economy.
Whwn you start is you have to hold your foot down on the gas pedal.  The
car does not burn any oil and runs fine once you get moving and warmed up.
In the past year I replaced the cap and rotor, plugs, wires, air filter,
ran numerous cans of chevron techron in it, and I also replaced a few
other small things.  I had a 94 toyota camry that ran like carp and was
rough starting, I replace the o2 sensor and all was well.    Any other
ideas Tiger?- Chuck, Seaside NJ

Your not wrong.  the exhaust manifold is a common location for O2
sensors.  I guess yours is just a little further down the pipe...
Marty